FMGE 2025 - Microbiology Practice Questions

Q: A 30-year-old man develops profuse vomiting and diarrhea within 3 hours of consuming pre-packaged salad and milk at a picnic. There is no fever. Which of the following is the most likely causative organism?

Staphylococcus aureus. ***Staphylococcus aureus***- This organism produces a **heat-stable enterotoxin** that, when ingested (preformed toxin), causes rapid symptoms within 1 to 6 hours, fitting the **3-hour onset** without requiring bacterial multiplication in the host.- Symptoms are primarily profound **nausea and vomiting**, accompanied by diarrhea, and characteristically **without fever**, as this is a food intoxication rather than an invasive infection.*Clostridium perfringens*- Symptoms usually manifest after a longer incubation period of **8 to 16 hours**, as the toxin is produced *in vivo* after the bacteria are ingested.- The illness is dominated by **watery diarrhea** and abdominal cramps; vomiting is typically mild or absent.*Salmonella enterica*- This organism causes an **invasive infection** (or requires prolonged multiplication) and thus has a longer incubation period, typically **6 hours to 3 days**.- Gastroenteritis caused by *Salmonella* almost always presents with associated systemic symptoms, most notably significant **fever**.*Bacillus cereus*- *B. cereus* causes two syndromes: the **diarrheal form** (slower onset) and the **emetic form** (rapid 1–6-hour onset), which is mostly associated with contaminated **fried rice**.- While the emetic form fits the timeline, *Staphylococcus aureus* is the most common cause of rapid, non-febrile illness affecting both the stomach (vomiting) and intestines (diarrhea) related to cold, packaged foods like salads or milk.

Q: Receptor used for the entry of HIV virus into the host cell is?

CCR5. ***CCR5*** - HIV initially binds to the **CD4** receptor on the surface of T-helper cells, which triggers a conformational change in the viral envelope protein gp120. - This change exposes a binding site for a coreceptor, which is typically the chemokine receptor **CCR5** (or **CXCR4**), facilitating the fusion of the viral envelope with the host cell membrane and allowing viral entry. *CCR7* - **CCR7** is a chemokine receptor primarily involved in the homing of lymphocytes and dendritic cells to secondary lymphoid organs like lymph nodes. - It is not utilized by HIV as a coreceptor for entry into host cells. *CD6* - **CD6** is a cell surface receptor found on T cells that functions as a costimulatory molecule in T-cell activation and adhesion. - It plays no role in the binding or entry process of the HIV virus. *CD8* - **CD8** is a coreceptor molecule that defines cytotoxic T lymphocytes, which are essential for killing virally infected cells. - HIV primarily infects **CD4+** cells, not CD8+ cells, and CD8 does not serve as a viral entry receptor.

Q: Which organism is most commonly responsible for gas gangrene?

Clostridium perfringens. ***Clostridium perfringens*** - This **anaerobic, Gram-positive** rod is the most common cause of gas gangrene (or **myonecrosis**) due to its rapid production of potent toxins, especially $\alpha$-toxin (a **lecithinase**). - It is found ubiquitously in soil and the GI tract and typically infects deep penetrating wounds, leading to rapid tissue destruction and gas formation (crepitus). *Clostridium difficile* - This organism primarily causes **antibiotic-associated diarrhea** and **pseudomembranous colitis** due to the production of Toxin A (enterotoxin) and Toxin B (cytotoxin). - It is rarely involved in tissue infections or gas gangrene. *Clostridium tetani* - This bacterium is the causative agent of **tetanus**, a neurological disorder characterized by muscle spasms and locked jaw. - Tetanus symptoms are due to the production of the neurotoxin **tetanospasmin**, which blocks inhibitory neurotransmitters. *Clostridium botulinum* - This organism produces the potent **botulinum toxin**, which causes **flaccid paralysis** (botulism) by preventing the release of acetylcholine at the neuromuscular junction. - It is associated with food poisoning or infant botulism, not typically gas gangrene.

Q: A woman was bitten by a rabid dog. The dog developed symptoms and died within a week. What is the best method for post-mortem diagnosis of rabies in the dog?

DFAT. ***DFAT (Direct Fluorescent Antibody Test)*** - The **Direct Fluorescent Antibody Test (DFAT)** is the **gold standard** for post-mortem diagnosis of rabies in animals - It detects rabies viral antigen in **brain tissue** with high sensitivity and specificity - Provides **rapid results** (within hours) and is the WHO-recommended method for definitive diagnosis - Used worldwide as the standard confirmatory test for rabies in suspected animals *Detection of Negri bodies* - **Negri bodies** are pathognomonic eosinophilic cytoplasmic inclusions found in neurons (especially hippocampal pyramidal cells) - Detection has **low sensitivity** (only 50-80% of cases) and requires expert interpretation - While specific when present, their absence does not rule out rabies - Not reliable as the primary diagnostic method *Seller's staining* - **Seller's staining** is a classical histological technique specifically used to visualize Negri bodies - It is an **outdated method** with the same limitations as general Negri body detection - Has been largely replaced by more sensitive and specific methods like DFAT - Lower sensitivity compared to immunofluorescence techniques *Brain biopsy* - While brain tissue is the ideal specimen for rabies diagnosis, **brain biopsy** is not performed post-mortem - Post-mortem examination involves direct collection of brain tissue, not biopsy - For antemortem human diagnosis, less invasive methods (skin biopsy from nape of neck, corneal impressions) are preferred over brain biopsy

Q: A 3-month-old child was admitted with pneumonia. The microscopy shows the following image. Choose the correct organism.

Influenza. ***Influenza*** - The image displays a virus with two key surface glycoproteins: **Hemagglutinin (HA)** and **Neuraminidase (NA)**. These proteins are the defining structural features of the Influenza virus and are crucial for its entry into and exit from host cells. - Influenza virus is a well-known cause of respiratory illnesses, including severe **pneumonia**, particularly in high-risk groups such as infants, the elderly, and immunocompromised individuals. *Rotavirus* - Rotavirus is the leading cause of severe **gastroenteritis** (diarrhea and vomiting) in infants and young children; it is not a primary cause of pneumonia. - It is a **non-enveloped** virus with a distinct wheel-like appearance (hence the name "rota"), which is structurally different from the enveloped virus shown in the diagram. *Hepatitis virus* - Hepatitis viruses (A, B, C, etc.) are hepatotropic, meaning they primarily infect and cause inflammation of the **liver (hepatitis)**, and are not associated with respiratory infections like pneumonia. - Their viral structures and surface antigens are different; for example, Hepatitis B virus has **HBsAg** on its surface, not HA and NA. *Respiratory syncytial virus* - While RSV is a very common cause of **pneumonia** and **bronchiolitis** in infants, its viral structure differs from the one depicted. - RSV belongs to the *Paramyxoviridae* family and has **Fusion (F)** and **Attachment (G)** proteins on its surface, not Hemagglutinin and Neuraminidase.

Q: Identify the microorganism shown in the image.

Entamoeba histolytica. ***Entamoeba histolytica*** - The image displays the **trophozoite** stage, characterized by a single nucleus with a central, dot-like **karyosome**. - A key diagnostic feature seen here is the presence of ingested **red blood cells** within the cytoplasm, a phenomenon known as **erythrophagocytosis**, which indicates invasive disease. *Giardia lamblia* - *Giardia* trophozoites are pear-shaped and **binucleated**, often described as having a face-like appearance, which is distinct from the single nucleus seen in the image. - They are flagellated protozoa and do not phagocytose red blood cells. *Shigella dysenteriae* - *Shigella* is a **bacterium**, not a protozoan, and would appear as a small, rod-shaped organism under a microscope. - It lacks the complex eukaryotic structures, such as a distinct nucleus and cytoplasmic inclusions, that are visible in the provided image. *Campylobacter jejuni* - This is a **bacterium** known for its characteristic curved, S-shaped, or "gull-wing" morphology. - As a prokaryote, it is much smaller and structurally simpler than the large amoeboid parasite shown.

Question 1

A 30-year-old man develops profuse vomiting and diarrhea within 3 hours of consuming pre-packaged salad and milk at a picnic. There is no fever. Which of the following is the most likely causative organism?

Accepted Answer

Staphylococcus aureus

Answer

Bacillus cereus

Answer

Salmonella enterica

Answer

Clostridium perfringens

Question 2

Receptor used for the entry of HIV virus into the host cell is?

Accepted Answer

CCR5

Answer

CD8

Answer

CD6

Answer

CCR7

Question 3

Which organism is most commonly responsible for gas gangrene?

Accepted Answer

Clostridium perfringens

Answer

Clostridium difficile

Answer

Clostridium tetani

Answer

Clostridium botulinum

Question 4

A woman was bitten by a rabid dog. The dog developed symptoms and died within a week. What is the best method for post-mortem diagnosis of rabies in the dog?

Accepted Answer

DFAT

Answer

Detection of Negri bodies

Answer

Brain biopsy

Answer

Seller’s staining

Question 5

A 3-month-old child was admitted with pneumonia. The microscopy shows the following image. Choose the correct organism.

Accepted Answer

Influenza

Answer

Rotavirus

Answer

Hepatitis virus

Answer

Respiratory syncytial virus

Question 6

Identify the microorganism shown in the image.

Accepted Answer

Entamoeba histolytica

Answer

Giardia lamblia

Answer

Campylobacter jejuni

Answer

Shigella dysenteriae

FMGE 2025 — Microbiology