NEET-PG 2025 — Microbiology
10 Previous Year Questions with Answers & Explanations
A patient presents with irregular swelling over the foot, multiple discharging sinuses, and black granules. A KOH mount is performed on the discharge. What is the most likely observation?
A 5-year-old child presents with nocturnal perianal itching. The image below shows the organism identified on an adhesive tape test. What is the most likely causative agent?
A patient presents with a history of chronic meningitis. Laboratory findings show Gram-positive, filamentous branching bacteria, Positive ZN stain, Growth on paraffin bait culture. Which of the following is the most likely causative organism?
Group A Streptococcus is the most common cause of bacterial pharyngitis in school-aged children. Which of the following bacterial components is primarily responsible for its attachment to fibronectin on the epithelial lining of the pharynx?
In a village, several people developed dysentery after consuming raw milk. On laboratory examination, gram-negative, curved rods with polymorphonuclear infiltration were found in stool samples. Which of the following is the most likely causative organism?
A patient presents with low-grade fever, chronic cough, and weight loss. Fungal culture from respiratory secretions shows the following organism with characteristic tuberculate macroconidia on microscopy. What is the most likely diagnosis?
A young adult presents with facial pain and painful vesicular lesions in the mouth. Tzanck smear reveals multinucleated giant cells with intranuclear inclusions. What is the most likely causative organism?
A patient presents with recurrent infections with Neisseria gonorrhoeae. Which of the following investigations is most appropriate to evaluate the underlying immunodeficiency?
A farmer presents with an ulcerative skin lesion showing signs of necrosis. Smear from the ulcer stained with polychrome methylene blue reveals capsulated bacilli that are McFadyean reaction positive. What is the most likely causative organism?
A middle-aged man from an endemic region presents with progressive swelling of the lower limb. A peripheral blood smear shows the following structure. What is the most likely cause of his limb swelling?
NEET-PG 2025 - Microbiology NEET-PG Practice Questions and MCQs
Question 1: A patient presents with irregular swelling over the foot, multiple discharging sinuses, and black granules. A KOH mount is performed on the discharge. What is the most likely observation?
- A. Slender dematiaceous fungi (Correct Answer)
- B. Yeast
- C. Septate hyphae 4-5
- D. Arthrospores
Explanation: ***Slender dematiaceous fungi*** - The clinical triad of irregular swelling, multiple discharging sinuses, and the presence of **black granules** strongly suggests **black-grained eumycetoma** (e.g., caused by *Madurella mycetomatis*). - KOH mount of the black granule reveals densely packed, brown-to-black pigmented (**dematiaceous**) septate hyphal elements and chlamydospores, consistent with slender dematiaceous fungi. *Arthrospores* - Arthrospores are typically seen in superficial infections like **dermatophytosis** (e.g., Tinea capitis, *Trichophyton*) or are characteristic of the parasitic phase of *Coccidioides immitis*. - They are not the primary microscopic finding within the tissue granule of mycetoma. *Yeast* - Yeast forms are characteristic of systemic infections like **Candidiasis**, Cryptococcosis, or Histoplasmosis, or are seen as sclerotic (Medlar) bodies in **Chromoblastomycosis**. - Mycetoma granules are formed by highly organized masses of filamentous hyphae, not unicellular yeast. *Septate hyphae 4-5* - This describes the general morphology and width of hyphae seen in common molds like **Aspergillus** or *Fusarium*. - This description is incomplete for black-grained mycetoma as it omits the distinctive feature of the fungus being pigmented (**dematiaceous**), which is critical for identifying the organism within the black granule.
Question 2: A 5-year-old child presents with nocturnal perianal itching. The image below shows the organism identified on an adhesive tape test. What is the most likely causative agent?
- A. Hymenolepis nana
- B. Trichuris trichiura
- C. Enterobius vermicularis (Correct Answer)
- D. Ancylostoma duodenale
Explanation: ***Enterobius vermicularis*** - The clinical presentation of **nocturnal perianal itching** (pruritus ani) in a child is highly characteristic of **pinworm infection** (enterobiasis). - The image shows a classic **D-shaped** or planoconvex egg, frequently found on the **adhesive tape test**, which are key identifiers for *Enterobius vermicularis*. *Ancylostoma duodenale* - This parasite causes **hookworm disease**, typically manifesting as **iron-deficiency anemia** and rarely perianal itching. - *Ancylostoma* eggs are oval, thin-shelled, and typically found in the **stool**, not via a tape test. *Hymenolepis nana* - This is the **dwarf tapeworm**, which usually causes mild or asymptomatic infection, though heavy infections can cause abdominal discomfort. - The eggs are small, oval, and possess **polar filaments** between the shell and the oncosphere, which are absent in the image. *Trichuris trichiura* - This parasite, the **whipworm**, causes trichuriasis, which presents with bloody diarrhea and occasionally **rectal prolapse** in heavy infections. - *Trichuris* eggs are distinctively **barrel-shaped** with two prominent bipolar mucoid plugs at each end.
Question 3: A patient presents with a history of chronic meningitis. Laboratory findings show Gram-positive, filamentous branching bacteria, Positive ZN stain, Growth on paraffin bait culture. Which of the following is the most likely causative organism?
- A. Actinomyces israelii
- B. Mycobacterium tuberculosis
- C. Cryptococcus neoformans
- D. Nocardia asteroides (Correct Answer)
Explanation: ***Nocardia asteroides*** - This organism is characterized by **Gram-positive, filamentous, branching bacteria** which are also typically **weakly acid-fast** (Positive ZN stain), differentiating it from Actinomyces. - The ability to utilize paraffin as a sole carbon source and grow on **paraffin bait culture** is a classical diagnostic method specifically employed for identifying **Nocardia** species, fitting the clinical picture of chronic meningitis (Nocardiosis). *Actinomyces israelii* - Although it is a **Gram-positive, filamentous, branching bacterium**, *Actinomyces* is strictly **anaerobic** and is typically **non-acid fast** (ZN negative), which contradicts the lab finding. - It commonly causes chronic suppurative infections like lumpy jaw, often involving **sulfur granules**, and is not diagnosed via paraffin culture methods. *Mycobacterium tuberculosis* - This organism is a major cause of **chronic meningitis** and is **strongly acid-fast** (ZN positive), but its morphology is that of a simple rod (**bacillus**), not a **filamentous branching** structure. - It requires specific media like **Lowenstein-Jensen (LJ) medium** for culture and is not known to grow on paraffin bait. *Cryptococcus neoformans* - This is an encapsulated **yeast (fungus)**, not a bacterium, and therefore does not show **filamentous branching** or Gram-positive bacterial morphology. - Diagnosis is typically made via **India ink stain** or detection of **Cryptococcal antigen** in CSF, not ZN staining or paraffin bait culture.
Question 4: Group A Streptococcus is the most common cause of bacterial pharyngitis in school-aged children. Which of the following bacterial components is primarily responsible for its attachment to fibronectin on the epithelial lining of the pharynx?
- A. Lipoteichoic acid (Correct Answer)
- B. Capsule
- C. Flagella
- D. Lipoprotein
Explanation: ***Lipoteichoic acid*** - **Lipoteichoic acid (LTA)**, along with the **M protein**, serves as a key adhesin for Group A Streptococcus, mediating attachment by binding to **fibronectin** receptors on pharyngeal epithelial cells. - This adherence mechanism is essential for the initial colonization and subsequent establishment of **streptococcal pharyngitis**. *Lipoprotein* - **Lipoproteins** are structurally diverse cell components, but they are not recognized as the primary molecular component mediating the specific attachment of GAS to **fibronectin**. - They often reside in the cell membrane and typically function in **nutrient transport** or signaling pathways. *Capsule* - The **hyaluronic acid capsule** of Group A Streptococcus mainly functions as an **anti-phagocytic factor** against host immune cells, preventing ingestion. - While important for virulence, it is not the molecule that directly facilitates initial adherence to the **fibronectin** layer of the host epithelium. *Flagella* - **Flagella** are organelles primarily responsible for **bacterial motility** (movement). - *Streptococcus pyogenes* (Group A Streptococcus) is a **non-motile** organism and therefore lacks flagella.
Question 5: In a village, several people developed dysentery after consuming raw milk. On laboratory examination, gram-negative, curved rods with polymorphonuclear infiltration were found in stool samples. Which of the following is the most likely causative organism?
- A. Staphylococcus aureus
- B. Vibrio parahaemolyticus
- C. Campylobacter jejuni (Correct Answer)
- D. Clostridium perfringens
Explanation: ***Campylobacter jejuni*** - This organism is a **Gram-negative, curved or S-shaped rod** (seagull appearance) that causes inflammatory diarrhea (**dysentery**) and is the most common antecedent of **Guillain-Barré Syndrome**. - The association of dysentery with consumption of **raw milk** is a classic epidemiological feature of *C. jejuni* infection, explaining the outbreak. *Clostridium perfringens* - This is a **Gram-positive, spore-forming rod** and causes toxin-mediated food poisoning (Type A) characterized primarily by watery diarrhea, not typically dysentery. - The source is usually contaminated meat and gravies, and the stool sample would show a **Gram-positive rod**, not a curved Gram-negative rod. *Staphylococcus aureus* - This organism is a **Gram-positive coccus** that causes food intoxication with rapid onset vomiting being the prominent feature, not dysentery. - The pathology is preformed toxin-mediated, and stool analysis would reveal **cocci**, not curved rods, and lacks the PMN infiltration typical of dysentery. *Vibrio parahaemolyticus* - While it is a **Gram-negative curved rod**, it is strongly associated with the consumption of **raw or undercooked seafood**, especially shellfish. - Clinically, it usually causes watery diarrhea, and raw milk is not its typical vehicle of transmission for large outbreaks.
Question 6: A patient presents with low-grade fever, chronic cough, and weight loss. Fungal culture from respiratory secretions shows the following organism with characteristic tuberculate macroconidia on microscopy. What is the most likely diagnosis?
- A. Histoplasmosis (Correct Answer)
- B. Cryptococcosis
- C. Coccidioidomycosis
- D. Blastomycosis
Explanation: ***Histoplasmosis*** - The image displayed shows characteristic large, spiny, thick-walled structures known as **tuberculate macroconidia**, which are pathognomonic for the mold phase of ***Histoplasma capsulatum*** grown at 25°C. - The clinical presentation of chronic cough, low-grade fever, and weight loss is typical of chronic pulmonary **Histoplasmosis** in immunocompetent individuals and often mimics tuberculosis. *Blastomycosis* - The yeast form of *Blastomyces dermatitidis* is characterized by having distinctive **broad-based budding** and large size (8-15 µm). - The mold phase of *Blastomyces* produces oval to pear-shaped microconidia on short or long stalks, not the tuberculate macroconidia seen in the picture. *Cryptococcosis* - *Cryptococcus neoformans* is a yeast encapsulated with a polysaccharide capsule and typically demonstrates smooth, **narrow-based budding** without forming true hyphae or macroconidia in culture. - Although it can cause pulmonary disease, its yeast morphology and lack of dimorphism differentiate it from the organism shown. *Coccidioidomycosis* - The mold form of *Coccidioides immitis* produces thin-walled, barrel-shaped structures called **arthroconidia**, which are highly infectious. - The tissue phase is defined by large **spherules containing endospores**, a morphology that is distinctly different from the tuberculate macroconidia shown.
Question 7: A young adult presents with facial pain and painful vesicular lesions in the mouth. Tzanck smear reveals multinucleated giant cells with intranuclear inclusions. What is the most likely causative organism?
- A. Adenovirus
- B. Herpes simplex virus (Correct Answer)
- C. Cytomegalovirus
- D. Epstein-Barr virus
Explanation: ***Herpes simplex virus*** - The presentation of **painful vesicular lesions** in the mouth and **facial pain** strongly suggests primary (e.g., gingivostomatitis) or reactivated herpes infection. - The **Tzanck smear** showing **multinucleated giant cells** with **intranuclear inclusions** is characteristic of the Herpesviridae family, confirming the clinical suspicion of HSV. *Adenovirus* - Adenovirus typically causes **pharyngoconjunctival fever**, acute respiratory disease, or viral conjunctivitis, not localized vesicular lesions in the mouth. - It does not produce the hallmark cytopathic effect of **multinucleated giant cells** on Tzanck smear. *Cytomegalovirus* - CMV usually presents as a mild **mononucleosis-like syndrome** in healthy adults or causes severe systemic disease in the immunocompromised. - CMV histology is defined by large intranuclear **"owl's eye" inclusions**, a finding different from the multinucleated giant cells seen on Tzanck smear. *Epstein-Barr virus* - EBV causes **Infectious Mononucleosis**, characterized by fever, lymphadenopathy, and pharyngitis, not primarily painful vesicular eruptions. - Although part of the herpes family, EBV typically causes unique lymphoid pathology and **Hairy Leukoplakia**, not the cytopathic changes seen on Tzanck smear.
Question 8: A patient presents with recurrent infections with Neisseria gonorrhoeae. Which of the following investigations is most appropriate to evaluate the underlying immunodeficiency?
- A. Nitroblue tetrazolium test
- B. Terminal complement (C5-C9) assay (Correct Answer)
- C. Quantitative immunoglobulin levels
- D. C1 esterase inhibitor assay
Explanation: ***Terminal complement (C5-C9) assay*** - Recurrent infections with *Neisseria* species (gonorrhoeae or meningitidis) are the classic presentation of a deficiency in the **Terminal Complement Components (C5-C9)**. - These components assemble the **Membrane Attack Complex (MAC)**, which is crucial for the lysis and killing of the thin cell walls of Neisserial organisms. *C1 esterase inhibitor assay* - This assay is used to diagnose **Hereditary Angioedema (HAE)**, which presents with recurrent episodes of swelling, not recurrent bacterial infections like *N. gonorrhoeae*. - HAE involves uncontrolled activation of the complement and kinin pathways, not failure to eliminate bacteria. *Quantitative immunoglobulin levels* - Measuring **IgG, IgA, and IgM** levels evaluates humoral immunity, typically diagnosing conditions like **Common Variable Immunodeficiency (CVID)**. - Antibody deficiencies usually lead to recurrent sinopulmonary infections with encapsulated bacteria (*Strep pneumoniae*, *H flu*), not specifically *Neisseria*. *Nitroblue tetrazolium test* - This test, or the modern DHR assay, screens for **Chronic Granulomatous Disease (CGD)**, a defect in phagocyte function (NADPH oxidase). - CGD patients suffer from severe, deep-seated infections and abscesses caused by **catalase-positive organisms** (e.g., *Staph aureus*, *Aspergillus*), not complement-dependent issues like recurrent *Neisseria*.
Question 9: A farmer presents with an ulcerative skin lesion showing signs of necrosis. Smear from the ulcer stained with polychrome methylene blue reveals capsulated bacilli that are McFadyean reaction positive. What is the most likely causative organism?
- A. Yersinia pestis
- B. Francisella tularensis
- C. Bacillus anthracis (Correct Answer)
- D. Clostridium perfringens
Explanation: ***Bacillus anthracis*** - The combination of a necrotic skin ulcer (eschar) in an exposed individual (farmer) and the specific laboratory findings is diagnostic for **Cutaneous Anthrax**. - The **McFadyean reaction** is a specific test for *Bacillus anthracis*, where the D-glutamic acid polypeptide capsule is stained **pink/purple** against the blue bacilli using **Polychrome Methylene Blue (PMB)** stain. *Clostridium perfringens* - This organism typically causes **gas gangrene** (clostridial myonecrosis) characterized by severe tissue destruction and gas production (crepitus), not usually a single necrotic ulcer. - Although it is a Gram-positive bacillus, it is an **anaerobe**, unlike the aerobic *Bacillus anthracis*, and the McFadyean reaction is not used for its identification. *Yersinia pestis* - This Gram-negative organism causes **plague** and typically presents with painful, swollen lymph nodes called **buboes** (buboes plague). - It is classified as a small, pleomorphic, non-capsulated bacillus and does not exhibit a positive McFadyean reaction with Polychrome Methylene Blue. *Francisella tularensis* - This causative agent of **tularemia** often causes an ulceroglandular syndrome, but it is a small, Gram-negative coccobacillus and does not have the macroscopic capsule visualized by the McFadyean reaction. - It requires specialized culture media like **cysteine-glucose-blood agar** due to its fastidious nature, differentiating it from *Bacillus* species.
Question 10: A middle-aged man from an endemic region presents with progressive swelling of the lower limb. A peripheral blood smear shows the following structure. What is the most likely cause of his limb swelling?
- A. Hypoalbuminemia
- B. Lymphatic obstruction (Correct Answer)
- C. Hypoproteinemia
- D. Increased hydrostatic pressure
Explanation: ***Lymphatic obstruction*** - The image displays **microfilariae** (suggesting **Lymphatic Filariasis**), where the adult worms settle in the lymphatics, physically blocking flow. - This obstruction results in chronic, progressive edema (known as **lymphedema** or **elephantiasis**) due to the failure of lymphatic drainage in the affected limb. *Hypoalbuminemia* - Edema from **hypoalbuminemia** is due to low plasma oncotic pressure and typically results in generalized, pitting edema, which is not characteristic of chronic, localized filarial swelling. - While malnutrition might co-exist, the core pathology shown by the presence of **microfilariae** points specifically to mechanical lymphatic blockage. *Hypoproteinemia* - **Hypoproteinemia** is a general cause of edema via reduced **oncotic pressure**, leading to fluid shift into the interstitial spaces, usually causing generalized rather than localized swelling. - This systemic cause does not account for the specific parasitic damage to the lymphatics demonstrated by the presence of the **filarial worms**. *increased hydrostatic pressure* - Increased hydrostatic pressure (e.g., in conditions like DVT or CHF) leads to fluid leakage from capillaries, causing pitting edema. - The root cause here is the parasitic **destruction and obstruction of lymph nodes and channels**, which specifically impairs lymph return, not capillary hydrostatic pressure regulation.