MicroBiology MCQs with Answer and Explnation | Chapter 32

Answer: Escherichia coli

In the context of pyelonephritis with shaking chills and fever, the most likely etiology is Escherichia coli. Escherichia coli is a common causative agent of urinary tract infections (UTIs) and pyelonephritis. It is a Gram-negative bacterium that frequently colonizes the gastrointestinal tract and can ascend the urinary tract, leading to infections.

The other options are incorrect:

• Group B Streptococcus (GBS): While GBS can cause urosepsis in newborns and pregnant women, it’s not a typical cause of pyelonephritis in adults like in this case.
• Staphylococcus aureus (S. aureus): S. aureus typically causes skin and soft tissue infections and is less likely to be associated with pyelonephritis.
• Streptococcus pyogenes (Strep pyogenes): This Gram-positive cocci typically causes strep throat and skin infections, and not pyelonephritis.

Answer: Streptococcus pneumoniae

In the context of meningitis with CSF (cerebrospinal fluid) growing Gram-positive diplococci, the most likely causative agent is Streptococcus pneumoniae. Streptococcus pneumoniae is a Gram-positive diplococcus and a common pathogen associated with bacterial meningitis in adults.

The other options are incorrect:

• Neisseria meningitidis: While also Gram-negative diplococci, N. meningitidis is less common in adults and more frequent in young children and adolescents.
• Staphylococcus aureus: This Gram-positive cocci, usually in clusters, causes less common meningitis, primarily associated with healthcare settings or complications from other infections.
• Salmonella typhi: This Gram-negative rod causes typhoid fever, not meningitis.

Answer: Papovavirus

Among the options provided, the viral family known to be causally associated with tumor formation is Papovavirus. Papovaviruses include the subfamily Papillomaviridae and Polyomaviridae. Members of the Polyomaviridae family, specifically certain polyomaviruses such as Simian Virus 40 (SV40), have been experimentally shown to induce tumors in laboratory animals. Additionally, human polyomaviruses, including JC virus and BK virus, have been associated with diseases in immunocompromised individuals.

The other options are incorrect:

• Flavivirus: Flaviviruses, such as the hepatitis C virus and West Nile virus, are not known to be causally associated with tumor formation. They are primarily associated with infections and specific diseases.
• Paramyxovirus: Paramyxoviruses, including measles virus and mumps virus, are not typically associated with tumor formation. They primarily cause respiratory and systemic infections.
• Polyoma virus: While Polyoma viruses (a subfamily of Papovaviridae) can be associated with tumor formation, this option is correct. Certain polyomaviruses, such as SV40, have been studied for their tumorigenic potential.

Answer: Gallbladder

Salmonella typhi has a strong affinity for the biliary system, particularly the gallbladder. Its unique ability to invade and persist within gallbladder epithelial cells makes it a prime location for colonization during the carrier state.

The other options are incorrect:

• Blood: While Salmonella typhi can enter the bloodstream during acute typhoid fever, the carrier state often involves gallbladder colonization without persistent bacteremia.
• Kidney: Although rarely, Salmonella typhi can persist in the urinary tract, the gallbladder remains the predominant site during asymptomatic carriage.
• Lungs: Salmonella typhi primarily targets the gastrointestinal tract and biliary system, not the lungs

Answer: Halophilic – Salmonella typhi

The mismatched pair in the options provided is “Halophilic – Salmonella typhi.” Salmonella typhi is not typically halophilic. Halophiles are microorganisms that thrive in high-salt environments, and Salmonella typhi is more associated with causing typhoid fever in the intestines rather than salt-rich environments.

The other options are incorrect:

• Severe dehydration – Vibrio cholerae: Vibrio cholerae is associated with causing cholera, a diarrheal disease that can lead to severe dehydration due to the profuse watery diarrhea it induces.
• Multi-drug resistance – Pseudomonas aeruginosa: Pseudomonas aeruginosa is known for its ability to develop multi-drug resistance, making it challenging to treat infections caused by this bacterium.
• Coagulase positive – Staphylococcus aureus: Staphylococcus aureus is coagulase positive, meaning it can produce the enzyme coagulase, which causes the coagulation of blood plasma. This characteristic is correctly matched.

Answer: Group A Streptococcus (S. pyogenes)

In a 12-hour-old newborn with a temperature of 103°F and blood culture growing Gram-positive cocci in chains, the most likely causative agent is Group A Streptococcus (S. pyogenes). Group A Streptococcus is a Gram-positive bacterium that can cause early-onset neonatal sepsis, and it is known for its chain formation when observed under a microscope.

The other options are incorrect:

• Staphylococcus aureus: Staphylococcus aureus is a Gram-positive bacterium, but it typically appears in clusters rather than chains. It can cause various infections, but it is not the most likely causative agent in this scenario.
• Neisseria meningitidis: Neisseria meningitidis is a Gram-negative bacterium that is more commonly associated with meningitis in older infants and children. It is not the most likely cause of sepsis in a 12-hour-old newborn.
• Mycobacterium tuberculosis: Mycobacterium tuberculosis is a slow-growing acid-fast bacillus that causes tuberculosis. It is not the likely cause of early-onset sepsis in a newborn, and it does not appear as Gram-positive cocci in chains.

Answer: Polysaccharide capsule

The primary factor associated with Streptococcus pneumoniae is the presence of a polysaccharide capsule. The capsule is a crucial virulence factor for S. pneumoniae as it helps the bacterium evade the host’s immune system, particularly phagocytosis. The capsule inhibits opsonization and phagocytosis, allowing the bacteria to resist clearance by the immune cells.

The other options are incorrect:

• Pilli: These hair-like appendages on bacterial surfaces facilitate attachment to host cells, but do not play a primary role in S. pneumoniae virulence compared to the capsule.
• Polypeptide capsule: This term is not accurate for S. pneumoniae, as their capsule is composed of polysaccharides, not polypeptides.
• Coagulase: This enzyme produced by some Staphylococcal species, not by S. pneumoniae, contributes to clot formation and tissue invasion, but not as a primary virulence factor in pneumococcal infections.

Answer: M protein

The virulence factors of Staphylococcus aureus include Coagulase, Enterotoxin, and Protein A. However, M protein is not a virulence factor associated with Staphylococcus aureus; it is primarily a virulence factor of Group A Streptococcus (Streptococcus pyogenes).

The other options are incorrect:

• Coagulase: Coagulase is an enzyme produced by Staphylococcus aureus, and it plays a role in the formation of fibrin clots, which helps the bacterium evade the host immune system.
• Enterotoxin: Staphylococcus aureus produces enterotoxins, which are exotoxins responsible for causing food poisoning when contaminated food is consumed.
• Protein A: Protein A is a virulence factor produced by Staphylococcus aureus. It binds to the Fc region of antibodies, preventing opsonization and phagocytosis by immune cells.

Answer: Staphylococcus aureus

This Gram-positive bacterium frequently produces heat-stable enterotoxins like staphylococcal enterotoxins (SEs) that survive cooking and remain active within food. Ingesting food contaminated with preformed SEs leads to rapid onset (2-8 hours) of food poisoning symptoms like nausea, vomiting, abdominal cramps, and diarrhea.

The other options are incorrect:

• Staphylococcus epidermidis: While closely related to S. aureus, S. epidermidis is much less likely to produce enterotoxins and primarily causes skin and healthcare-associated infections.
• Staphylococcus saprophyticus: This species mainly associates with urinary tract infections and isn’t typically associated with foodborne illness or enterotoxin production.
• Streptococcus pneumoniae: This Gram-positive bacterium is a respiratory pathogen responsible for pneumonia and meningitis, not associated with food poisoning or enterotoxin production.

Answer: Positive hepatitis B surface antigen

In the scenario described, the finding that would implicate hepatitis B as the etiology of the nurse’s jaundice is a “Positive hepatitis B surface antigen (HBsAg).” The presence of HBsAg in the blood indicates an active hepatitis B virus (HBV) infection. The nurse’s needle stick injury, coupled with the patient’s history of illicit intravenous drug use, increases the risk of exposure to hepatitis B.

The other options are incorrect:

• Positive anti-hepatitis B surface antibody (anti-HBs): This indicates past infection or immunity to hepatitis B, not necessarily an active case. In the nurse’s scenario, one month is too short for anti-HBs to develop after a potential exposure.
• Positive anti-hepatitis B-core antibody (anti-HBc): This can be positive in both acute and chronic infection but also persists after recovery. It alone is not conclusive for an active case.
• Positive anti-hepatitis A antibody: This indicates past infection or immunity to hepatitis A, not hepatitis B. It wouldn’t explain the nurse’s symptoms.

Answer: Intravenous drug (needles)

HCV is primarily transmitted through blood-to-blood contact. Sharing contaminated needles or equipment used for intravenous drug use is a common mode of transmission.

The other options are incorrect:

• Fecal-oral: HCV is not typically transmitted through feces. Although the virus can be present in stool, the amount is usually too small and the conditions needed for fecal-oral transmission are uncommon.
• Fomite: While HCV can survive on surfaces for a short time, transmission through fomites (contaminated objects) is rare. The virus needs direct contact with open wounds or mucous membranes to enter the body.
• Sexual transmission: While possible, sexual transmission of HCV is relatively uncommon. The risk increases with certain factors like multiple sexual partners, unprotected sex, and presence of other sexually transmitted infections. However, compared to intravenous drug use, the likelihood of transmission through sexual activity is significantly lower.

Answer: Level of HIV-1 RNA in plasma

The amount of HIV-1 RNA in the plasma, also known as viral load, is the strongest indicator of viral replication and immune system damage. High viral load signifies active HIV replication and increased risk of disease progression and AIDS-related complications. Therefore, it serves as the most predictive factor for HIV-1 patient prognosis.

The other options are incorrect:

• Degree of lymphadenopathy: Enlarged lymph nodes (lymphadenopathy) can be a symptom of HIV infection, but it’s not a specific or reliable predictor of prognosis. The severity and duration of lymphadenopathy can vary greatly between individuals and may not directly correlate with viral load or immune function.
• CD4+:CD8+ cell ratio: While the ratio of CD4+ (helper T) cells to CD8+ (cytotoxic T) cells is important for immune function, it’s a less direct measure of viral activity compared to viral load. Low CD4+:CD8+ ratio indicates compromised immunity, but the degree of decline isn’t as sensitive to changes in viral load as compared to the actual viral load itself.
• Rate of decline in anti-HIV antibody: Anti-HIV antibodies are produced by the body in response to the virus, but their decline doesn’t directly reflect viral activity or disease progression. Antibody levels can fluctuate and may not accurately reflect current viral load or prognosis.

Answer: Cytomegalovirus

Cytomegalovirus is a member of the herpesvirus family and is particularly opportunistic in individuals with compromised immune systems, such as those with advanced HIV infection. CMV retinitis is a common manifestation in these patients and can lead to blindness if not treated.

The other options are incorrect:

• Candida albicans: While Candida albicans can cause various infections in HIV patients, it’s not typically associated with blindness.
• Mycobacterium tuberculosis: Although tuberculosis can cause severe complications in HIV-positive individuals, including vision loss, it’s not the primary cause of blindness directly attributable to HIV infection.
• Neisseria meningitidis: This bacterium is responsible for meningitis, an inflammation of the membranes surrounding the brain and spinal cord. While meningitis can lead to vision problems, it’s not a common cause of blindness in HIV patients.

Answer: Anogenital

HPV infections are a major risk factor for the development of anogenital cancers, including cervical, anal, and genital cancers. Certain high-risk HPV types, such as HPV 16 and 18, are particularly linked to the development of these cancers.

The other options are incorrect:

• Breast: HPV is not commonly associated with breast cancer. Breast cancer is more often linked to other factors, and the presence of HPV in breast cancer is not a prominent feature.
• Lung: While smoking and certain viruses (like HPV) can contribute to lung cancer, HPV is not the primary or most common cause of lung cancer. Tobacco use is a major risk factor for lung cancer.
• Prostate: HPV is generally not considered a major factor in the development of prostate cancer. Prostate cancer is influenced by factors such as age, family history, and hormonal changes, but it is not typically associated with HPV infection.

Answer: Destruction of infected cells

Poliovirus primarily infects and destroys motor neurons in the spinal cord, leading to muscle weakness and paralysis. The destruction of these cells is responsible for the characteristic symptoms of polio.

The other options are incorrect:

• Persistent viremia: While viremia (presence of the virus in the bloodstream) can occur during poliovirus infection, the main pathologic effect is not due to persistent viremia but rather the destruction of infected cells.
• Immune complex formation: Immune complex formation is not the primary pathologic effect of poliovirus infection. The destruction of infected cells and the resulting neurological damage are the key aspects of polio pathology.
• Aseptic meningitis: Poliovirus can cause aseptic meningitis, but this is not the primary pathologic effect. The main pathology involves the destruction of motor neurons, leading to muscle weakness and paralysis.

Answer: It induces local intestinal immunity.

The oral polio vaccine (OPV) offers a key advantage over the inactivated polio vaccine (IPV) thanks to its ability to induce local intestinal immunity. This means that OPV not only stimulates antibody production in the bloodstream but also triggers immune responses directly in the gut, where poliovirus initially replicates. This additional layer of protection helps prevent the virus from multiplying and shedding in stool, ultimately interrupting its transmission chain.

The other options are incorrect:

• It can be given to immunocompromised patients: The live attenuated nature of the oral polio vaccine makes it unsuitable for immunocompromised individuals, as it could cause vaccine-associated disease. In such cases, inactivated polio vaccine is preferred.
• It is not associated with vaccine-related cases of poliomyelitis: While the oral polio vaccine has been associated with rare cases of vaccine-derived poliovirus, it is still considered safer than wild-type poliovirus. Inactivated polio vaccine is less likely to cause vaccine-related cases of poliomyelitis.
• It is easily administered as a series of multiple injections: Oral polio vaccine is administered orally and does not require multiple injections. In contrast, inactivated polio vaccine is given through intramuscular injections.

Answer: Respiratory syncytial virus

Unlike the other listed paramyxoviruses, RSV does not possess a hemagglutinin (HA) attachment protein. Hemagglutinin plays a crucial role in viral attachment and entry into host cells. Instead, RSV utilizes a different attachment protein called G glycoprotein (G protein) for binding to host cell receptors.

The other options are incorrect:

• Parainfluenza virus: All four types of parainfluenza virus (1, 2, 3, and 4) possess Hemagglutinin-neuraminidase (HN) glycoproteins that function as both attachment and neuraminidase proteins.
• Mumps virus: Mumps virus also has HN glycoproteins similar to parainfluenza viruses, serving both attachment and neuraminidase functions.
• Measles virus: Measles virus utilizes hemagglutinin (H) protein distinct from HN but still functioning as an attachment protein.

Answer: Motile

Escherichia coli is generally motile, possessing flagella that enable movement, while Klebsiella species are typically non-motile. Motility is an important characteristic that distinguishes these two genera.

The other options are incorrect:

• Gram-negative: Both Escherichia coli and Klebsiella are Gram-negative bacteria. This characteristic is not a point of difference between them.
• Non-sporogenous: Both Escherichia coli and Klebsiella are non-sporogenous, meaning they do not form spores. This similarity does not differentiate them.
• Non-lactose fermenting: Escherichia coli is known for its ability to ferment lactose, while Klebsiella is also capable of lactose fermentation. Therefore, lactose fermentation is not a distinguishing feature between them.

Answer: Sporangium

Candida albicans is a dimorphic fungus, meaning it can exist in both yeast and hyphal forms depending on environmental conditions. It can also form pseudohyphae, which are chains of elongated yeast cells. However, it does not produce sporangia. Sporangia are structures associated with the reproductive processes of certain fungi but are not part of the typical morphology of Candida albicans.

The other options are incorrect:

• Yeast: Candida albicans exists in a yeast form, characterized by single-celled, oval or round structures. This is the most common form observed in clinical settings.
• Hyphae: Candida albicans can also form hyphae, elongated filamentous structures that allow it to invade and penetrate tissues. These hyphae can branch and contribute to the organism’s virulence.
• Pseudohyphae: Pseudohyphae are intermediate structures between yeast and true hyphae, appearing elongated but lacking the internal compartmentalization of true hyphae. Candida albicans can also form pseudohyphae, contributing to its adaptability and colonization potential.

Answer: An adult worm passed during a bowl movement

The presence of adult worms in the stool is a definitive diagnostic feature for ascariasis. Ascaris lumbricoides, the roundworm that causes ascariasis, resides in the small intestine of infected individuals, and mature worms are expelled in the feces.

The other options are incorrect:

• An eosinophilia in a differential WBC count: Eosinophilia (an increase in eosinophil white blood cells) can occur in response to various parasitic infections, including ascariasis. However, it is not specific to ascariasis and may be seen in other parasitic infections or allergic conditions.
• Motile larvae in a stool sample: While the detection of motile larvae in a stool sample is suggestive of parasitic infection, it may not be specific to ascariasis. The observation of adult worms provides a more definitive diagnosis.
• Larvae in radiography of lungs: Ascariasis involves a migratory phase where larvae travel through the bloodstream to the lungs. However, the larvae are not typically visible in standard radiography of the lungs. The presence of adult worms in the stool is a more direct diagnostic approach.

Answer: Damage to cytoplasmic membrane

Polymyxin primarily acts by damaging the cytoplasmic membrane of susceptible bacteria. Its cationic amphiphilic structure disrupts the phospholipid arrangement in the membrane, leading to increased permeability and leakage of essential intracellular components. This ultimately results in cell death.

The other options are incorrect:

• Inhibition of cell-wall synthesis: While some antibiotics target cell wall synthesis, polymyxin does not affect this process. Gram-negative bacteria, the primary target of polymyxin, already have an outer membrane barrier in addition to the cell wall, making them less susceptible to this mechanism.
• Inhibition of nucleic acid and protein synthesis: Polymyxin does not directly inhibit the translation or transcription of genetic material. Its action predominantly focuses on disruption of the cytoplasmic membrane.
• Inhibition of specific enzyme systems: Although some antibiotics exert their effect by inhibiting specific enzymes within bacterial cells, this is not the primary mode of action for polymyxin. Its membranolytic activity is the key mechanism contributing to its bactericidal effect.

Answer: Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that commonly causes meningitis in immunocompromised individuals. The latex agglutination test is often used to detect capsular polysaccharide antigens of Cryptococcus in the CSF, aiding in the diagnosis of cryptococcal meningitis.

The other options are incorrect:

• Aspergillus fumigatus: Aspergillus fumigatus is a fungus that can cause invasive aspergillosis, but it is not a common cause of meningitis. It is more often associated with respiratory infections.
• Toxoplasma gondii: Toxoplasma gondii is a protozoan parasite that can cause toxoplasmosis, but it is not a typical cause of meningitis. Toxoplasmosis is more commonly associated with encephalitis in immunocompromised individuals.
• Nocardia asteroides: Nocardia asteroides is a bacterium that can cause nocardiosis, which may involve the central nervous system. However, it is not a common cause of meningitis, and the latex agglutination test is not specific for Nocardia.

Answer: Mebendazole

Mebendazole is an anthelmintic medication that is effective against intestinal parasitic worms, including Ascaris lumbricoides, the roundworm that causes ascariasis. Mebendazole works by inhibiting the worms’ ability to absorb glucose, leading to their immobilization and eventual expulsion from the body.

The other options are incorrect:

• Metronidazole: Metronidazole is an antibiotic with activity against certain bacteria and parasites, but it is not the drug of choice for treating ascariasis. Mebendazole is more specific for helminthic infections.
• Penicillin: Penicillin is an antibiotic effective against bacteria, and it does not have activity against parasitic worms like Ascaris. Mebendazole is the appropriate choice for treating ascariasis.
• Niclosamide: Niclosamide is an anthelmintic medication, but it is not the first-line treatment for ascariasis. Mebendazole is more commonly used and is considered more effective against Ascaris lumbricoides.

Answer: Oocyst

Cryptosporidium, the protozoan parasite causing cryptosporidiosis, has a life cycle that involves the production of oocysts. These oocysts are shed in the feces of infected individuals and are resistant to environmental conditions. Identification of oocysts in stool samples is a key diagnostic indicator for cryptosporidiosis.

The other options are incorrect:

• Cyst: Cryptosporidium doesn’t form cysts. Their infectious stage is the oocyst.
• Merozoites: Merozoites are a stage of some other parasites like Plasmodium (malaria). Cryptosporidium has sporozoites within its oocyst.
• Egg: Eggs are a stage of intestinal worms like roundworms or whipworms. Cryptosporidium is a coccidian parasite with a different life cycle and doesn’t form eggs.

Answer: Snail

Snails serve as intermediate hosts for trematodes (flukes). In the life cycle of trematodes, eggs are released by the adult parasites in the definitive host (often a vertebrate). The eggs hatch into larvae (miracidia) in water, and these larvae infect snails. Inside the snails, the larvae undergo further development into cercariae, which are released into the water. Cercariae can then infect the definitive host, where they mature into adult trematodes.

The other options are incorrect:

• Flea: Fleas are not intermediate hosts for trematodes. Fleas are more commonly associated with the transmission of certain types of parasites like tapeworms (cestodes).
• Mosquito: Mosquitoes are not intermediate hosts for trematodes. They are primarily associated with the transmission of parasites such as Plasmodium, which causes malaria.
• Sand fly: Sand flies are not intermediate hosts for trematodes. They are known for transmitting parasites like Leishmania, which causes leishmaniasis, but they are not part of the life cycle of trematodes.

Answer: 350,000

The formula for calculating the bacteria/ml is:

Bacteria/ml

Number of colonies
×
Dilution factor
Volume plated
Bacteria/ml=
Volume plated
Number of colonies×Dilution factor
​In this case:

Bacteria/ml

35
×
100

0.01

350
,
000
Bacteria/ml=
0.01
35×100
​
=350,000

The other options are incorrect:

• 35,000: This is incorrect because it does not account for the dilution factor. The correct calculation involves multiplying the number of colonies by the dilution factor.
• 3,500: This is incorrect because it does not consider the dilution factor. The correct calculation involves multiplying the number of colonies by the dilution factor.
• 350: This is incorrect because it does not account for the dilution factor. The correct calculation involves multiplying the number of colonies by the dilution factor.

Answer: Water is not polluted with E. coli

The MPN test is a statistical method used to estimate the concentration of viable microorganisms, such as E. coli, in a water sample. The results 0, 1, 3 indicate that the series of tubes tested showed no significant growth (0 tubes with growth), growth in 1 tube, and growth in 3 tubes. This pattern is consistent with a low probability of E. coli contamination, and the water is likely not polluted with E. coli.

The other options are incorrect:

• Water is potable: While the results suggest low or no E. coli contamination, the term “potable” refers to water that is safe for drinking. The absence of E. coli does not guarantee the absence of other contaminants, and additional tests may be needed for a comprehensive assessment of water safety.
• Error in collecting water sample: The results provided are related to the presence of E. coli rather than errors in sample collection. The pattern 0, 1, 3 indicates a specific microbial count.
• Dilution of media is incorrect: The MPN method accounts for dilutions, and the results are interpreted based on statistical probabilities. The pattern 0, 1, 3 is not indicative of an issue with the dilution of the media.

Answer: Lactose

The ability to ferment lactose is a key characteristic used to detect coliforms, a group of Gram-negative bacteria commonly found in the intestines of humans and animals. Coliforms are often used as indicator organisms for fecal contamination in water and food samples.

The other options are incorrect:

• Glucose: While many coliforms can ferment glucose, it is not specific enough to distinguish them from other types of bacteria. Many non-coliform bacteria can also ferment glucose.
• Mannitol: Some coliforms, particularly certain members of the Klebsiella species, can ferment mannitol. However, many coliforms, including Escherichia coli, the most common coliform, do not ferment mannitol. Therefore, it is not a reliable indicator for all coliforms.
• Dextrose: Dextrose (another term for glucose) also suffers from the same limitations as glucose and cannot differentiate coliforms from other bacteria.

Answer: Liver homogenate

The Ames test is a bacterial mutagenicity assay used to assess the mutagenic potential of chemical substances. To make the bacterial system more comparable to the mammalian metabolic activation system, liver homogenate is added. The liver is a key organ in mammals for metabolizing and activating various chemicals, and the inclusion of liver homogenate in the test helps simulate the metabolic conditions in mammals.

The other options are incorrect:

• Kidney homogenate: Kidney homogenate is not typically used in the Ames test for metabolic activation. The liver is the primary organ involved in the metabolism of many substances.
• Spleen homogenate: Spleen homogenate is not commonly used in the Ames test for metabolic activation. The liver is the primary focus due to its role in metabolizing xenobiotics.
• Blood homogenate: While blood may contain enzymes, the liver is a more specific and critical organ for metabolizing various substances. Liver homogenate is the standard addition to simulate mammalian metabolic conditions.

Answer: A. Presence of mycolic acid

The mycolic acid present in the cell wall of Mycobacterium tuberculosis, the bacterium causing tuberculosis, contributes to its acid-fastness. Mycolic acid is a long-chain fatty acid that imparts resistance to decolorization by acid-alcohol during staining procedures, such as the Ziehl-Neelsen stain or the acid-fast stain.

The other options are incorrect:

• Integrity of cell wall: While the integrity of the cell wall is important, the specific attribute responsible for acid-fastness in tubercle bacilli is the presence of mycolic acid.
• Both of above: This is partially correct. While mycolic acid is a key contributor to acid-fastness, the integrity of the cell wall as a whole is not the primary factor.
• Cell Membrane: The acid-fastness of tubercle bacilli is primarily related to the mycolic acid in the cell wall, not the cell membrane.

Answer: Asymptomatic infection

In the majority of cases, poliovirus infection is asymptomatic or causes only mild, flu-like symptoms. Only a small percentage of infected individuals develop more severe manifestations, such as aseptic meningitis or flaccid paralysis.

The other options are incorrect:

• Flaccid paralysis of ≥1 extremities: While poliovirus can cause flaccid paralysis, the majority of infected individuals do not experience this severe symptom. Paralysis is a less common outcome.
• Aseptic meningitis: Aseptic meningitis can occur in some cases of poliovirus infection, but it is not the predominant symptom in the majority of non-immunized patients.
• Muscle spasms and pain: Muscle spasms and pain can be associated with poliovirus infection, but they are not the most common symptoms in the majority of cases.

Answer: Capsular polysaccharides

The capsular polysaccharide layer is a virulence factor for many bacteria, including Streptococcus pneumoniae (pneumococci). This polysaccharide capsule inhibits the recognition and phagocytosis of the bacteria by the host’s immune cells, such as macrophages. The capsule helps the bacteria evade the host’s immune response, contributing to the pathogenicity of pneumococci.

The other options are incorrect:

• Haemolysin: Haemolysin is a toxin that can lyse red blood cells, but it is not primarily involved in protecting pneumococci against phagocytosis.
• Hyaluronidase: Hyaluronidase is an enzyme that breaks down hyaluronic acid, a component of connective tissues. While it facilitates the spread of bacteria through tissues, it is not a major factor in protecting pneumococci against phagocytosis.
• Necrotizing toxin: Necrotizing toxins cause tissue damage but are not specifically associated with protecting pneumococci against phagocytosis. They can contribute to the overall pathogenicity of certain bacteria.

Answer: 0.5 -1 % sugar

Bacterial culture media often include a carbon source to support the growth of bacteria. The concentration of sugar in routine culture media is commonly within the range of 0.5 – 1%. This concentration provides a suitable energy and carbon source for bacterial metabolism.

The other options are incorrect:

• 2% sugar: A concentration of 2% sugar is relatively high for routine bacterial culture media. Such a high concentration may be used in specialized media but is not typical for routine culture.
• 0.1 – 0.5% sugar: This concentration is on the lower side and may not provide sufficient nutrients for the growth of a variety of bacteria in routine culture. A higher concentration is more common.
• <0.1% sugar: A sugar concentration less than 0.1% is very low and might not support robust bacterial growth in routine culture conditions. A slightly higher concentration is usually used for routine bacterial culture media.

Answer: Repetitive occurrence of antigenic determinants

The concept of repetitive occurrence of antigenic determinants, also known as multivalency or multivalence, refers to the presence of multiple copies of the same antigenic determinant on a single antigen molecule. This repetitive structure enhances the ability of the antigen to cross-link with multiple antibody molecules simultaneously, leading to a more potent immune response. Multivalency increases the efficiency of immune recognition and response.

The other options are incorrect:

• Increased number of epitopes: While a larger antigen may have more epitopes (regions that bind to antibodies), simply having more epitopes doesn’t necessarily make the antigen more potent. The key factor is the density and arrangement of these epitopes.
• Larger antigen determinants: Larger individual epitopes might not enhance immunogenicity compared to smaller, well-defined epitopes. The specific amino acid sequence and presentation of the antigenic determinant are crucial for strong antibody binding.
• None of the above: This option is incorrect because repetitive occurrence of antigenic determinants is a well-established mechanism for enhancing antigen potency.

Answer: Sabouraud agar

Sabouraud agar is a specialized culture medium used for the isolation and cultivation of fungi. It has a lower pH and a higher glucose concentration than general-purpose media like nutrient agar, making it more selective for fungi. The composition of Sabouraud agar provides an environment that is conducive to the growth of fungi while inhibiting the growth of bacteria.

The other options are incorrect:

• Nutrient broth: Nutrient broth is a liquid medium that supports the growth of a wide range of microorganisms, including bacteria and fungi. However, it is not as selective for fungi as Sabouraud agar.
• Nutrient agar: Nutrient agar is a solid medium that supports the growth of both bacteria and fungi. While it is a general-purpose medium, Sabouraud agar is more specific for fungal isolation.
• MaConkey medium: MaConkey medium is selective for the isolation of Gram-negative bacteria based on lactose fermentation. It is not suitable for fungal culture.

Answer: Minimum inhibitory concentration (MIC)

The MIC is the lowest concentration of an antimicrobial agent (such as a drug or antibiotic) that inhibits the visible growth of a microorganism after a specific incubation period. It is a crucial parameter in determining the effectiveness of an antimicrobial agent against a particular bacterium.

The other options are incorrect:

• Minimum Bacterial Concentration (MBC): The MBC is the minimum concentration of an antimicrobial agent required to kill the bacterium, not just inhibit its growth. It involves subculturing from the MIC-containing medium to a drug-free medium to determine bacterial viability.
• Lf Dose: “Lf dose” is not a standard term related to the determination of drug concentration affecting bacterial growth. It does not correspond to MIC.
• LD50: LD50 (Lethal Dose 50) is a measure of the lethal dose of a substance required to kill 50% of a population in a specific time frame. It is not related to inhibitory concentrations for bacterial growth.

Answer: Common cold

The common cold is caused by various viruses, primarily rhinoviruses and coronaviruses. It is highly prevalent and easily transmitted from person to person, making it the most common infectious viral disease in humans. The symptoms include a runny or stuffy nose, sneezing, coughing, and a sore throat.

The other options are incorrect:

• Rabies: Rabies is a viral disease caused by the rabies virus. It is relatively rare in comparison to the common cold and is typically transmitted through the bite of an infected animal.
• Chickenpox: Chickenpox is caused by the varicella-zoster virus. While it is a common childhood illness, the common cold is more widespread and occurs more frequently.
• Measles: Measles is caused by the measles virus. Although it can be highly contagious, the common cold is still more prevalent globally.

Answer: Ability to cause abnormalities in developing fetus

Teratogenicity refers to the ability of a substance to cause structural or functional abnormalities in the developing fetus when exposure occurs during pregnancy. Chemical contaminants in water with teratogenic properties can adversely affect fetal development and lead to birth defects.

The other options are incorrect:

• Ability to cause abortion in pregnant women: While some chemical contaminants can increase the risk of miscarriage, causing abnormalities in the developing fetus is a broader and more accurate definition of teratogenicity.
• Ability to cause heritable changes in DNA: This refers to mutagenicity, not teratogenicity. Mutagens affect germ cells (sperm and egg cells), potentially causing changes in genes passed on to offspring, while teratogens influence the development of the fetus in the womb.
• Ability to cause cancerous growth: This refers to carcinogenicity, a different phenomenon where substances trigger uncontrolled cell growth and tumor formation.

Answer: Gene therapy

Gene therapy involves introducing genetic material into a patient’s cells to correct or replace a defective gene. DNA/RNA tumor viruses are used as vectors to deliver the normal gene into the target cells. The goal is to restore normal gene function and alleviate the effects of genetic disorders.

The other options are incorrect:

• Active Immunotherapy: Active immunotherapy involves stimulating the patient’s immune system to recognize and eliminate abnormal cells, such as cancer cells. It is not focused on replacing or correcting genes.
• Genetic Counseling: Genetic counseling involves providing information and support to individuals and families dealing with genetic disorders. It is not a therapeutic intervention but rather a counseling service.
• Interferon Therapy: Interferon therapy involves the use of interferons, proteins that play a role in the immune response, to treat certain viral infections and cancers. It is not specifically targeted at replacing or correcting mutant genes.

Answer: Cytomegalovirus

Cytomegalovirus (CMV) infection is associated with the formation of intracytoplasmic inclusion bodies. These inclusion bodies consist of viral particles and cellular components and can be observed in various tissues and cell types infected by CMV.

The other options are incorrect:

• Echovirus: Echovirus does not typically cause the formation of intracytoplasmic inclusion bodies. It is an enterovirus associated with various clinical manifestations, including febrile illness and aseptic meningitis.
• Rabies virus: Rabies virus infection is characterized by the presence of Negri bodies, which are intracytoplasmic inclusion bodies, but they are found in neurons, not in a wide range of cell types.
• Influenza virus: Influenza virus infection does not lead to the formation of intracytoplasmic inclusion bodies. Instead, it is associated with nuclear inclusions known as viral replication complexes.

Answer: Transfer of DNA into bacterium

Transformation is a process in which a bacterium takes up free DNA from its environment and incorporates it into its own genome. This ability is utilized in various molecular biology techniques, including the introduction of foreign genes into bacterial cells for cloning or genetic engineering purposes.

The other options are incorrect:

• Infection of bacterium with phage: This is more broadly termed bacteriophage adsorption or infection, not transformation. While phage DNA enters the bacterium during infection, it may not integrate into the bacterial chromosome, and the outcome can vary depending on the phage type (lytic or lysogenic).
• Transfer of phage from one bacterium to another: This process is called transduction, a specific type of horizontal gene transfer mediated by phages. While it involves DNA movement, it’s distinct from transformation, which focuses on uptake and integration of DNA into the recipient bacterium’s own genome.
• Transfer of DNA to mammalian cell: This falls under the broader category of gene transfer or gene therapy in eukaryotes. Transformation specifically refers to bacterial DNA uptake and integration.

Answer: Colony count or pore plate method

The colony count or pore plate method involves diluting a sample and spreading it onto a solid agar medium to allow individual bacterial cells to form colonies. Each visible colony represents a viable bacterial cell that was capable of forming a colony under the given conditions. This method provides an accurate count of viable cells.

The other options are incorrect:

• Direct microscopic count: Direct microscopic count involves counting bacteria under a microscope, but it does not distinguish between live and dead cells. Live and dead cells both contribute to the count, making it less specific for viable cells.
• Photometric measurement of turbidity: Turbidity measurements are based on the cloudiness of a liquid culture, which can be influenced by both live and dead cells. It is not as specific for viable cells as the colony count method.
• Agglutination with specific antiserum: Agglutination assays are used for detecting specific antibodies or antigens, but they do not provide a direct count of viable bacteria. They are more focused on detecting specific interactions.

Answer: Hypotrophs

Heterotrophs are organisms that rely on external sources for their organic carbon and energy. In the context of the question, these organisms lack their own metabolic systems and instead utilize the metabolic machinery of the host cell to obtain nutrients and energy.

The other options are incorrect:

• Autotrophs: Autotrophs are organisms that can produce their own organic molecules, typically through processes like photosynthesis or chemosynthesis. They do not depend on the host cell for energy.
• Hypotrophs: “Hypotroph” is not a standard term in microbiology or biology. The correct term for organisms lacking their own metabolic system is “heterotroph.”
• L-forms: L-forms, or L-phase variants, are bacteria that have lost their cell walls. They are not necessarily organisms devoid of metabolic systems and are not the primary focus of the question.

Answer: Chloromycetin

Chloromycetin, also known as chloramphenicol, acts by inhibiting bacterial protein synthesis, specifically by binding to the bacterial ribosomes. It does not target the cell membrane.

The other options are incorrect:

• Colistin: Colistin is a polymyxin antibiotic that acts on the bacterial cell membrane. It disrupts the membrane structure, leading to leakage and cell death.
• Polymyxin: Polymyxin, like colistin, is a membrane-active antibiotic. It interacts with the bacterial cell membrane, causing damage and disrupting its integrity.
• Nystatin: Nystatin is an antifungal antibiotic that acts on fungal cell membranes by binding to ergosterol, a component of fungal cell membranes. It is not effective against bacterial cell membranes.

Answer: Distinct combining sites on the surface of a given antigen which is responsible for the specificity of the immune response

Epitopes are specific regions on the surface of an antigen molecule that can be recognized and bound by antibodies or T cell receptors. These interactions are crucial for triggering the immune response against the antigen.

The other options are incorrect:

• Effective number of reacting sites on the antigen: This term doesn’t accurately describe epitopes. While an antigen may have multiple epitopes, the “effective number” depends on factors like accessibility and antibody affinity, not just the number of sites.
• Precipitate formed when both antigen and antibody react in appropriate proportions: This refers to the visible clumping that occurs when antigens and antibodies bind, a consequence of the immune response but not the definition of an epitope itself.
• Three dimensional lattice structures formed by antigen antibody reaction: This describes the crystal-like structures that can form under certain conditions after antigen-antibody binding, but it’s not a general feature of epitopes or the immune response.

Answer: Five

Each IgM molecule is composed of five domains per monomer, making ten domains in total for a typical pentameric IgM structure.

The other options are incorrect:

• Four: While one chain of IgM has four constant domains, there are five domains in total when considering both V and C regions.
• Tow: This is likely a misspelling of “Two,” but IgM has five domains, not two.
• One: This only accounts for the V domain in one chain, neglecting the remaining C domains and the multimeric structure of IgM.

Answer: IgG

Only IgG antibodies can effectively cross the placenta and provide passive immunity to the fetus. This ability is crucial for protecting an infant before its own immune system fully develops. Other immunoglobulins, while important for various immune functions, cannot cross the placenta due to size or structural limitations.

The other options are incorrect:

• IgM: IgM is too large to cross the placenta. It is primarily found in the bloodstream and is the first antibody produced during an immune response.
• IgA: IgA is primarily found in mucosal secretions (such as saliva and breast milk) and does not readily cross the placenta.
• IgD:IgD is found on the surface of B cells and is involved in the activation of B cells. It does not have a significant role in crossing the placenta.

Answer: Guineaworm

While all four parasites listed can be present in water at some point in their life cycles, only Guineaworm can be directly transmitted and prevented by filtering water.

The other options are incorrect:

• Tapeworm: Tapeworm infections are primarily transmitted through the ingestion of contaminated food, not water. Filtering water is not a preventive measure for tapeworms.
• Roundworm: Roundworm infections, such as ascariasis caused by Ascaris lumbricoides, are transmitted through ingestion of contaminated soil or food, not water. Filtration is not a preventive measure for roundworms in water.
• Pinworm: Pinworm (Enterobius vermicularis) infections are also not transmitted through water. They are typically transmitted through the ingestion of infective eggs, often via contaminated hands or surfaces.

Answer: Ankylostoma braziliensis

Cutaneous larva migrans (CLM), also known as creeping eruption, is a skin infection caused by the larvae of hookworms, specifically Ankylostoma braziliensis. These larvae burrow into the skin and migrate, creating itchy, winding tracks.

The other options are incorrect:

• Ancylostoma duodenale: Ancylostoma duodenale is another hookworm species, but it is primarily associated with intestinal infections (hookworm disease) rather than cutaneous larva migrans.
• Toxocara canis: Toxocara canis is the roundworm of dogs, and human infection can occur through ingestion of contaminated soil. It is not associated with cutaneous larva migrans.
• Toxoplasma gondii: Toxoplasma gondii is a protozoan parasite that causes toxoplasmosis. It is not related to cutaneous larva migrans, which is caused by hookworm larvae.

Answer: Toxoplasmosis

Toxoplasmosis is caused by the protozoan parasite Toxoplasma gondii. This parasite has the ability to cross the placenta, leading to congenital infections if a pregnant woman becomes infected. Congenital toxoplasmosis can have serious consequences for the developing fetus.

The other options are incorrect:

• Malarial parasite: Malarial parasites, such as Plasmodium species, do not typically cross the placenta. The transmission of malaria during pregnancy is associated with the sequestration of infected red blood cells in the placenta but does not involve direct crossing of the parasite.
• Tuberculosis: Mycobacterium tuberculosis, the bacterium causing tuberculosis, does not typically cross the placenta. Vertical transmission of tuberculosis usually occurs after birth through respiratory droplets.
• Amoebiasis: Amoebiasis, caused by the protozoan parasite Entamoeba histolytica, is not known for crossing the placenta. Infection is usually acquired through ingestion of contaminated food or water.