NEET-PG 2021 - Microbiology PYQs Practice Questions

Q: Name the parasite whose microfilariae have a sheath and no nuclei at the tail end.

Wuchereria bancrofti. **Wuchereria bancrofti** - **Wuchereria bancrofti** microfilariae are characterized by the presence of a **sheath** and a **clear tail end** that is devoid of nuclei. - This morphology is a key feature used in the microscopic differentiation of W. bancrofti from other filarial species. *Brugia malayi* - **Brugia malayi** microfilariae also have a **sheath**, but their tail end typically contains **two distinct nuclei** that are spaced apart. - They are generally shorter and have more kinky curves than W. bancrofti. *Loa loa* - **Loa loa** microfilariae possess a **sheath** but are distinguished by their **nuclei extending to the tip** of the tail, often in a more continuous pattern. - They are also known for their diurnal periodicity, which aids in diagnosis. *Onchocerca volvulus* - **Onchocerca volvulus** microfilariae are **unsheathed** and have nuclei that do **not extend to the tail tip**, leaving a distinct clear space. - They are typically found in the skin and subcutaneous tissue, rather than the blood. [Practice more Microbiology PYQs on OnCourse]

Q: A patient presents with a history of pastry intake causing food poisoning. What is the most likely causative agent?

Staphylococcus aureus. ***Staphylococcus aureus*** - *S. aureus* is a common cause of food poisoning linked to **creamy foods** like pastries, salads, and custards, as it produces **heat-stable enterotoxins** when allowed to proliferate. - The symptoms, typically rapid onset **nausea, vomiting**, and abdominal cramps, occur because of the **preformed toxins** in the food, not necessarily active infection. *Verotoxin-producing E. coli* - This strain, often **E. coli O157:H7**, is primarily associated with **undercooked beef** or contaminated produce, and typically causes **bloody diarrhea** and can lead to hemolytic uremic syndrome (HUS). - Its mechanism involves **verotoxins** directly damaging intestinal cells and blood vessels, which is different from the rapid, emetic-focused symptoms of *S. aureus* food poisoning. *Bacillus cereus* - *B. cereus* causes two main types of food poisoning: **emetic (vomiting)**, typically from **reheated rice**, and **diarrheal**, from meat products or vegetables. - While the emetic form can cause vomiting, it is most strongly associated with **rice dishes** and usually has a shorter incubation period than the diarrheal form, making *Staphylococcus aureus* a more classic cause for pastry-related outbreaks. *Enteroinvasive E. coli (EIEC)* - EIEC causes a disease similar to **shigellosis**, involving direct invasion of intestinal epithelial cells, leading to **bloody diarrhea** and fever. - It is typically spread through contaminated food and water and not specifically linked to pastry intake or characterized by the rapid onset emetic symptoms seen with preformed toxins. [Practice more Microbiology PYQs on OnCourse]

Q: A patient presented with 70% burns, and a sample was collected from the burn site. The image shows Gram-negative rods, and the suspected organism is an obligate aerobe. What is the most likely causative microbe?

Pseudomonas aeruginosa. ***Pseudomonas aeruginosa*** - The image shows **Gram-negative rods**, and the patient has extensive **burns**, making *Pseudomonas aeruginosa* a highly likely causative agent due to its common association with burn wound infections. - *Pseudomonas aeruginosa* is an **obligate aerobe** and thrives in moist environments, making it a frequent colonizer of burn wounds, which are large, often moist surfaces. *Neisseria meningitidis (Meningococcus)* - *Neisseria meningitidis* is a **Gram-negative coccus**, typically appearing as diplococci, not rods, on Gram stain. - While it can cause severe infections, it is primarily associated with **meningitis** and **sepsis**, not typically burn wound infections. *Streptococcus pneumoniae (Pneumococcus)* - *Streptococcus pneumoniae* is a **Gram-positive coccus**, appearing as lancet-shaped diplococci or short chains, which contradicts the Gram-negative rod morphology seen in the image. - It is a common cause of **pneumonia** and **otitis media**, not primarily associated with burn wound infections. *Streptococcus pyogenes* - *Streptococcus pyogenes* is a **Gram-positive coccus** that grows in chains, which is inconsistent with the Gram-negative rod morphology. - Although it can cause skin infections like cellulitis and impetigo, it is not a typical cause of **burn wound infections** in the way *Pseudomonas aeruginosa* is. [Practice more Microbiology PYQs on OnCourse]

Q: A truck driver presented with a painless, demarcated ulcer on the penis and inguinal lymphadenopathy. What is the best method to visualize the motility of the most likely causative agent?

Dark field microscopy. ***Dark field microscopy*** - The symptoms (painless, demarcated penile ulcer and inguinal lymphadenopathy) are highly suggestive of **primary syphilis**, caused by *Treponema pallidum*. - **Dark field microscopy** is the gold standard for visualizing the characteristic **corkscrew motility** of *T. pallidum* directly from lesion exudate. *Fluorescent microscopy* - This technique uses **fluorochromes** to stain structures and is often used in **immunofluorescence** assays to detect antibodies or antigens. - While useful for some microbial identification, it is not the primary method for visualizing the motility of *Treponema pallidum*. *Light microscopy* - Standard light microscopy has **insufficient resolution** to clearly visualize the thin, coiled spirochetes of *Treponema pallidum* or their motility. - The organisms are generally **too small and refractile** to be easily seen without specialized illumination. *Electron microscopy* - Provides extremely **high resolution** and is used for studying viral structures or detailed cellular ultrastructure. - It is **not practical** for routine clinical diagnosis, especially for live, motile bacteria, and is not used to observe motility. [Practice more Microbiology PYQs on OnCourse]

Q: A patient was suspected of having brucellosis. A serum sample was sent for a standard agglutination test, which was initially negative but became positive after dilution of the sample. What is the most likely reason for the initial negative test?

Prozone phenomenon. ***Correct: Prozone phenomenon*** - The **prozone phenomenon** occurs when there is a very high concentration of antibodies in the patient's serum, leading to the formation of small antigen-antibody complexes that do not agglutinate or precipitate. - Diluting the sample reduces the antibody concentration, allowing for optimal antigen-antibody lattice formation and visible agglutination. - This is the classic explanation for a **negative test becoming positive after dilution** in brucellosis serology. *Incorrect: Antigen antibody complexes* - While agglutination tests rely on the formation of **antigen-antibody complexes**, the initial negative result despite a positive finding after dilution indicates a specific issue with complex *visibility* or *stability* rather than the general presence of complexes. - This option is too general and doesn't explain why dilution would change the result from negative to positive. *Incorrect: Postzone phenomenon* - The **postzone phenomenon** occurs when there is an *excess of antigen* relative to antibody, leading to no visible agglutination. - In such a case, diluting the sample (which would reduce antigen concentration or keep antibody concentration too low) would typically *not* lead to a positive result; in fact, further dilution of antibodies would worsen the outcome. - Postzone is the opposite mechanism and would not be corrected by dilution. *Incorrect: Complement inactivation* - **Complement inactivation** is not directly relevant to the mechanism of agglutination tests, which primarily depend on direct antibody-antigen binding for visible clumping. - These tests do not typically require complement activity for their primary reaction, nor are they inhibited by complement inactivation. [Practice more Microbiology PYQs on OnCourse]

Question 1

Name the parasite whose microfilariae have a sheath and no nuclei at the tail end.

Accepted Answer

Wuchereria bancrofti

Answer

Brugia malayi

Answer

Loa loa

Answer

Onchocerca volvulus

Question 2

A patient presents with a history of pastry intake causing food poisoning. What is the most likely causative agent?

Accepted Answer

Staphylococcus aureus

Answer

Verotoxin-producing E. coli

Answer

Bacillus cereus

Answer

Enteroinvasive E. coli (EIEC)

Question 3

A patient presented with 70% burns, and a sample was collected from the burn site. The image shows Gram-negative rods, and the suspected organism is an obligate aerobe. What is the most likely causative microbe?

Accepted Answer

Pseudomonas aeruginosa

Answer

Neisseria meningitidis (Meningococcus)

Answer

Streptococcus pneumoniae (Pneumococcus)

Answer

Streptococcus pyogenes

Question 4

A truck driver presented with a painless, demarcated ulcer on the penis and inguinal lymphadenopathy. What is the best method to visualize the motility of the most likely causative agent?

Accepted Answer

Dark field microscopy

Answer

Fluorescent microscopy

Answer

Light microscopy

Answer

Electron microscopy

Question 5

A patient was suspected of having brucellosis. A serum sample was sent for a standard agglutination test, which was initially negative but became positive after dilution of the sample. What is the most likely reason for the initial negative test?

Accepted Answer

Prozone phenomenon

Answer

Antigen antibody complexes

Answer

Postzone phenomenon

Answer

Complement inactivation

Question 6

A patient on steroids presented with nocturnal cough and chronic urticaria. Bronchoalveolar lavage (BAL) staining was done, and the organism shown in the image was identified. What is the most likely organism?

Accepted Answer

Strongyloides stercoralis

Answer

Enterobius vermicularis

Answer

Capillaria philippinensis

Answer

Ancylostoma duodenale

Question 7

A child presented with bloody stools and abdominal pain. Which enrichment medium should be used for processing the fecal sample?

Accepted Answer

Selenite F broth

Answer

Blood agar

Answer

Alkaline peptone water

Answer

Muller Hinton Broth

Question 8

What is the vector for Leishmania, a parasite characterized by a prominent kinetoplast in its morphological forms?

Accepted Answer

Sand fly

Answer

Female Anopheles Mosquito

Answer

Triatomine bug

Answer

TseTse fly

Question 9

A child presented with bluish-white spots in the mouth followed by a rash. What is the genome of the most likely causative agent?

Accepted Answer

Enveloped virus with single-stranded RNA

Answer

Double stranded Naked RNA

Answer

Naked virus with single-stranded RNA

Answer

Double stranded Enveloped RNA

Question 10

A group of people ate patty late at night and experienced bouts of vomiting early in the morning. What is the most likely cause?

Accepted Answer

Staphylococcus aureus

Answer

Escherichia coli

Answer

Lactobacillus

Answer

Bacillus cereus

NEET-PG 2021 — Microbiology