For the diagnosis of typhus, an antigen of Proteus is used. What is this type of antigen called?
A 39-year-old patient with HIV and a CD4 count of 139 cells/μL presents with altered sensorium and impaired consciousness. CSF examination using an India ink preparation reveals a positive result. Which organism is most likely responsible?
A person came with the H/o thorn prick a week ago. A few days later, he developed ulcers along lymphatic drainage. Choose the correct organism.
A HIV-positive patient presents with chronic diarrhoea for 4 weeks. Stool microscopy shows oval/ elliptical cysts measuring 16 X 20 μm. Diagnosis is?
A girl, after returning from a picnic, complains of pain in her eyes. She had not maintained proper hygiene of her contact lenses. What would be the cause?
Which opportunistic pathogen demonstrates a “crushed ping pong ball appearance” on Gomori methenamine stain?
Which antibody is the first to appear and acts fastest during a primary (first) immune response to an infection?
Chocolate agar is primarily used for the isolation of which of the following organisms?
A farmer presents with fever, malaise, and diarrhea. There is a history of exposure to livestock. Which of the following organisms is most likely responsible for the illness?
A diabetic patient presents with whitish plaques in the oral cavity. A KOH mount confirms the presence of budding yeast with pseudohyphae. Which of the following is the most appropriate culture medium for isolation of this organism?
FMGE 2025 - Microbiology FMGE Practice Questions and MCQs
Question 11: For the diagnosis of typhus, an antigen of Proteus is used. What is this type of antigen called?
- A. Iso antigen
- B. Heterophile antigen (Correct Answer)
- C. Super antigen
- D. Sequestered antigen
Explanation: ***Heterophile antigen*** - These are antigens shared between different species, such as the antigenic determinants shared by *Rickettsia* (causing typhus) and certain strains of **Proteus** (used in the Weil-Felix test). - The diagnosis of typhus relies on the **Weil-Felix reaction**, which detects anti-rickettsial antibodies that cross-react and agglutinate with the non-motile *Proteus* antigens (e.g., **OX-19**). *Sequestered antigen* - This term refers to self-antigens that are normally hidden from the immune system (e.g., in the eye or testicles) but can cause **autoimmunity** if released (e.g., post-trauma). - They are involved in the loss of **self-tolerance** and are not used in diagnostic agglutination tests for external infections like typhus. *Super antigen* - These are powerful microbial toxins, such as **Toxic Shock Syndrome Toxin-1**, that cause massive, non-specific activation of T cells. - They bind directly to the outside of the MHC Class II molecules and the T-cell receptor (TCR) Vβ chain, leading to a dangerous **cytokine storm**, unrelated to the agglutination reaction. *Iso antigen* - Also called **alloantigens**, these are specific antigens found in some individuals of a species but not others (e.g., **blood group antigens**). - They are relevant to transfusion reactions and graft rejection but do not describe the cross-species reactivity utilized in the Weil-Felix test for typhus.
Question 12: A 39-year-old patient with HIV and a CD4 count of 139 cells/μL presents with altered sensorium and impaired consciousness. CSF examination using an India ink preparation reveals a positive result. Which organism is most likely responsible?
- A. Candida albicans
- B. Histoplasma capsulatum
- C. Cryptococcus neoformans (Correct Answer)
- D. Aspergillus fumigatus
Explanation: ***Cryptococcus neoformans***- The classic finding of a positive **India ink preparation** upon CSF examination indicates the large polysaccharide **capsule** characteristic of this yeast.- In patients with advanced HIV (**CD4 <200** cells/μL), *C. neoformans* is the most common cause of **fungal meningoencephalitis** presenting with altered mental status.*Aspergillus fumigatus*- CNS infection with *Aspergillus* usually manifests as **focal cerebral lesions** (abscesses) or hemorrhagic infarction due to angioinvasion, not typically diffuse meningitis.- Diagnosis generally relies on culturing the fungus, **histopathology** showing septate hyphae, or detecting **galactomannan antigen** in the serum or CSF.*Histoplasma capsulatum*- While it can cause disseminated infection in patients with AIDS, CNS involvement is less frequent than *Cryptococcus* and the yeast is **intracellular** (within macrophages).- Detection typically utilizes **Histoplasma antigen testing** in urine or serum, as the small yeasts do not possess the large capsule highlighted by India ink.*Candida albicans*- *Candida* is a common cause of superficial or disseminated infection, but isolated **Candida meningitis** is rare, usually seen after neurosurgery or hematogenous seeding.- *Candida* **lacks the prominent capsule** that would produce a positive result on an India ink preparation.
Question 13: A person came with the H/o thorn prick a week ago. A few days later, he developed ulcers along lymphatic drainage. Choose the correct organism.
- A. Sporothrix schenckii (Correct Answer)
- B. Trichophyton rubrum
- C. Coccidioides immitis
- D. Aspergillus flavus
Explanation: ***Sporothrix schenckii*** - This dimorphic fungus is introduced via cutaneous trauma, classically associated with exposure to vegetation (e.g., **thorn prick**, sphagnum moss), leading to the term **Gardener's disease**. - The characteristic presentation of a primary nodule followed by secondary subcutaneous nodules or ulcers tracking along the proximal lymphatic drainage is diagnostic of **lymphocutaneous sporotrichosis**. *Coccidioides immitis* - This fungus causes **Coccidioidomycosis** (Valley fever), generally acquired by inhaling arthroconidia, leading primarily to pulmonary infection. - While disseminated infection can involve the skin, it does not typically present as a localized inoculation lesion with subsequent **ascending lymphangitis** from a thorn prick. *Aspergillus flavus* - **Aspergillus** species usually cause infections in immunocompromised patients (e.g., invasive pulmonary aspergillosis) or superficial infections such as fungal keratitis. - It is not the organism characteristically associated with traumatic inoculation from plant material causing a **sporotrichoid pattern** of lymphatic spread. *Trichophyton rubrum* - This is a common **dermatophyte** that causes superficial cutaneous infections of the keratin layer, such as tinea (ringworm) of the skin, hair, or nails. - It lacks the invasive capability to establish a deep infection followed by **lymphatic involvement** and ulceration after subcutaneous inoculation.
Question 14: A HIV-positive patient presents with chronic diarrhoea for 4 weeks. Stool microscopy shows oval/ elliptical cysts measuring 16 X 20 μm. Diagnosis is?
- A. Cystoisospora (Correct Answer)
- B. Cryptosporidium
- C. Cyclospora
- D. Cryptococcus
Explanation: ***Cystoisospora*** - The finding of large, oval/ellipsoidal oocysts (typically **20-33 μm** long, range includes the 16 x 20 μm given) in the stool of an HIV patient with chronic diarrhea is diagnostic of **Cystoisospora belli** infection. - *Cystoisospora* causes chronic, profuse watery diarrhea in immunocompromised hosts (like those with **AIDS**) and is effectively treated with **trimethoprim-sulfamethoxazole (TMP-SMX)**. *Cryptosporidium* - This parasite is characterized by small, **spherical oocysts** measuring only **4–6 μm** in diameter, which is much smaller than the size reported (16 x 20 μm). - While *Cryptosporidium* is a common cause of chronic diarrhea in AIDS, the oocyst morphology does not match the description. *Cryptococcus* - **Cryptococcus** is a fungal pathogen (*C. neoformans*) primarily known for causing **meningoencephalitis** in HIV patients, not typically large oval/elliptical oocysts in stool causing chronic primary diarrhea. - Diagnosis relies on visualization of encapsulated yeast in CSF (e.g., **India ink stain**) or **cryptococcal antigen (CrAg)** testing. *Cyclospora* - *Cyclospora cayetanensis* oocysts are medium-sized and **spherical**, typically measuring **8–10 μm** in diameter. - Although it causes prolonged diarrhea in immunocompromised individuals, the oocyst shape (**oval/elliptical**) and size (16 x 20 μm) clearly distinguish it from *Cyclospora*.
Question 15: A girl, after returning from a picnic, complains of pain in her eyes. She had not maintained proper hygiene of her contact lenses. What would be the cause?
- A. Pseudomonas aeruginosa
- B. Herpes simplex virus
- C. Acanthamoeba (Correct Answer)
- D. Fusarium
Explanation: ***Acanthamoeba*** - **Acanthamoeba** is a free-living amoeba commonly found in water sources (tap water, lakes, swimming pools) and soil. - Poor contact lens hygiene, such as rinsing lenses or storing them in non-sterile solutions (like tap water, which might be encountered during a picnic or travel hygiene lapse), is a key risk factor for **Acanthamoeba keratitis (AK)**. - AK is characterized by severe ocular pain, often disproportionate to the clinical findings, and can lead to a pathognomonic **ring-shaped corneal infiltrate**. - The combination of **outdoor water exposure + contact lens use + poor hygiene** strongly suggests Acanthamoeba. *Pseudomonas aeruginosa* - **Pseudomonas aeruginosa** is the most common cause of **bacterial keratitis** in contact lens users, especially with improper hygiene. - It typically causes rapidly progressive, purulent corneal ulceration with a characteristic greenish discharge. - While very common in contact lens wearers, the clinical picture here (picnic setting with potential water exposure) more specifically points to a protozoal etiology. *Herpes simplex virus* - **Herpes simplex virus (HSV)** causes recurrent viral keratitis, typically presenting with characteristic **dendritic ulcers** on the cornea. - HSV transmission is via contact with infected bodily fluids or reactivation of latent virus, not through contact lens solution contamination. *Fusarium* - **Fusarium** is a fungus that can cause fungal keratitis in contact lens wearers, particularly with exposure to organic matter or contaminated lens solutions. - Fungal keratitis tends to have a more indolent course with feathery, elevated stromal infiltrates. - While possible in contact lens users, the specific epidemiological context and clinical pattern here favor Acanthamoeba over Fusarium.
Question 16: Which opportunistic pathogen demonstrates a “crushed ping pong ball appearance” on Gomori methenamine stain?
- A. Cryptococcus neoformans
- B. Pneumocystis jirovecii (Correct Answer)
- C. Aspergillus fumigatus
- D. Histoplasma capsulatum
Explanation: ***Pneumocystis jirovecii*** - This opportunistic fungus classically appears as collapsed, crescent-shaped cysts on **Gomori methenamine silver (GMS) stain**, giving it the pathognomonic **“crushed ping pong ball”** appearance. - It is a common cause of **Pneumocystis pneumonia (PCP)** in immunocompromised individuals, particularly those with advanced HIV infection, and is typically found in the alveolar spaces. *Histoplasma capsulatum* - This dimorphic fungus appears as small, oval yeasts that are characteristically found **intracellularly within macrophages**. - It does not form the collapsed cysts seen with *P. jirovecii* and lacks the “crushed ping pong ball” morphology. *Cryptococcus neoformans* - This yeast is distinguished by its thick **polysaccharide capsule**, which creates a clear halo around the organism on **India ink stain**. - While it can be stained with GMS, it appears as a budding yeast and does not exhibit the collapsed cyst morphology. *Aspergillus fumigatus* - This mold is identified by its characteristic **septate hyphae** that branch at **acute (45-degree) angles**. - It does not form cysts in tissue and has a filamentous structure, which is entirely different from the appearance of *P. jirovecii*.
Question 17: Which antibody is the first to appear and acts fastest during a primary (first) immune response to an infection?
- A. IgE
- B. IgA
- C. IgG
- D. IgM (Correct Answer)
Explanation: ***IgM***- **IgM** is the first antibody isotype produced and secreted by **plasma B cells** following initial exposure to an antigen (primary immune response).- Because it is a **pentamer** (five Y-shaped units), IgM has 10 binding sites, giving it superior efficiency in complement activation, agglutination, and acting as the immediate responder.*IgG*- **IgG** is the most abundant immunoglobulin in the serum and generally appears later than IgM during the primary response.- It is the predominant antibody during the **secondary immune response**, providing long-term immunity and crossing the **placenta**.*IgE*- **IgE** is associated primarily with the initiation of **Type I hypersensitivity reactions** (allergies) by binding to mast cells and basophils.- It is also critical for defense against **helminthic parasites** but is not the first antibody in standard acute infection.*IgA*- **IgA** functions primarily in **mucosal immunity**, being secreted as a dimer into milk, saliva, tears, and mucosal linings (e.g., gut and respiratory tract).- While vital for preventing pathogen entry, it does not lead the kinetics of the initial systemic antibody response.
Question 18: Chocolate agar is primarily used for the isolation of which of the following organisms?
- A. Listeria monocytogenes
- B. Streptococcus pneumoniae
- C. Escherichia coli
- D. Haemophilus influenzae (Correct Answer)
Explanation: ***Haemophilus influenzae*** - *H. influenzae* is a **fastidious** organism requiring **X factor (hemin)** and **V factor (NAD/NADH)** for growth, both of which are supplied by the lysed red blood cells in chocolate agar. - Chocolate agar is essentially **heated blood agar** where the red blood cells have lysed, releasing essential growth factors required by organisms like *Haemophilus spp*. *Listeria monocytogenes* - *Listeria monocytogenes* is a **non-fastidious** organism that typically grows well on routine sheep **Blood Agar** (often showing weak beta-hemolysis). - Isolation of *Listeria* often utilizes cold enrichment or specialized selective media like **Oxford agar** or **PALCAM agar**. *Streptococcus pneumoniae* - *S. pneumoniae* typically grows well on routine **Blood Agar**, where it exhibits characteristic **alpha-hemolysis** (partial lysis resulting in a green discoloration). - While it can grow on chocolate agar, it does not require the specific X and V factors released, making Blood Agar the standard medium for its isolation. *Escherichia coli* - *E. coli* is a non-fastidious, common enteric bacterium that grows easily on non-enriched media like **Nutrient agar**. - It is typically identified using selective and differential media such as **MacConkey agar** (lactose-fermenting pink colonies) or Eosin Methylene Blue (EMB) agar.
Question 19: A farmer presents with fever, malaise, and diarrhea. There is a history of exposure to livestock. Which of the following organisms is most likely responsible for the illness?
- A. Coxiella burnetii (Correct Answer)
- B. Rickettsia typhi
- C. Orientia tsutsugamushi
- D. Rickettsia rickettsii
Explanation: ***Coxiella burnetii*** - This organism causes **Q fever**, a common zoonosis transmitted primarily through aerosols generated from the births or products of infected livestock, such as **sheep, goats, and cattle**, which aligns with the patient being a farmer. - Acute Q fever typically presents with non-specific severe symptoms including high fever, headache, significant **malaise**, and sometimes gastrointestinal issues like **diarrhea**. *Rickettsia rickettsii* - This organism is responsible for **Rocky Mountain spotted fever (RMSF)**, which is associated with a history of **tick bite** exposure, not direct livestock aerosol exposure. - RMSF classically presents with a prominent rash, often **petechial**, starting on the wrists and ankles and spreading centripetally, which is a key differentiating feature. *Rickettsia typhi* - This pathogen causes **murine typhus** (endemic typhus), transmitted through exposure to rat fleas, making it less likely given the farmer's exposure to livestock. - Murine typhus generally causes a milder presentation compared to epidemic typhus, often presenting with fever, headache, and a **truncal rash**. *Orientia tsutsugamushi* - This organism causes **scrub typhus**, found primarily in the Asia-Pacific region and transmitted by the bite of a **chigger** (mite larva). - A distinguishing feature of scrub typhus is the presence of a painless **eschar** (black, necrotic lesion) at the site of the chigger bite.
Question 20: A diabetic patient presents with whitish plaques in the oral cavity. A KOH mount confirms the presence of budding yeast with pseudohyphae. Which of the following is the most appropriate culture medium for isolation of this organism?
- A. Chocolate agar
- B. Thayer-Martin medium
- C. Sabouraud Dextrose Agar (SDA) (Correct Answer)
- D. Loeffler's serum slope
Explanation: ***Sabouraud Dextrose Agar (SDA)*** - SDA is the standard mycological culture medium used for the isolation of fungi, such as the yeast **_Candida albicans_** described in the clinical scenario. - Its **low pH** and high **dextrose** concentration inhibit the growth of most bacteria, making it selective for fungi. *Chocolate agar* - This is an enriched medium containing lysed red blood cells, primarily used for cultivating fastidious bacteria like **_Haemophilus influenzae_** and **_Neisseria_** species. - It is not a selective medium for fungi and would allow the overgrowth of oral bacteria, complicating the isolation of yeast. *Thayer-Martin medium* - This is a selective agar containing antibiotics, specifically designed to isolate pathogenic **_Neisseria_** species (**_N. gonorrhoeae_** and **_N. meningitidis_**) from samples with mixed flora. - The antibiotics in this medium inhibit the growth of gram-positive bacteria, most gram-negative bacteria, and yeast, making it unsuitable for this purpose. *Loeffler's serum slope* - This is an enrichment medium used for the cultivation of **_Corynebacterium diphtheriae_**, the causative agent of diphtheria. - It promotes the development of characteristic **metachromatic granules** within the bacteria and is not used for fungal isolation.