FMGE 2025 - Microbiology Practice Questions

Q: For the diagnosis of typhus, an antigen of Proteus is used. What is this type of antigen called?

Heterophile antigen. ***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.

Q: 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?

Cryptococcus neoformans. ***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.

Q: 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.

Sporothrix schenckii. ***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.

Q: A HIV-positive patient presents with chronic diarrhoea for 4 weeks. Stool microscopy shows oval/ elliptical cysts measuring 16 X 20 μm. Diagnosis is?

Cystoisospora. ***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*.

Q: 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?

Acanthamoeba. ***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.

Q: Which opportunistic pathogen demonstrates a “crushed ping pong ball appearance” on Gomori methenamine stain?

Pneumocystis jirovecii. ***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*.

Q: Which antibody is the first to appear and acts fastest during a primary (first) immune response to an infection?

IgM. ***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.

Q: Chocolate agar is primarily used for the isolation of which of the following organisms?

Haemophilus influenzae. ***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.

Q: 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?

Coxiella burnetii. ***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.

Q: 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?

Sabouraud Dextrose Agar (SDA). ***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.

Question 1

For the diagnosis of typhus, an antigen of Proteus is used. What is this type of antigen called?

Accepted Answer

Heterophile antigen

Answer

Iso antigen

Answer

Super antigen

Answer

Sequestered antigen

Question 2

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?

Accepted Answer

Cryptococcus neoformans

Answer

Candida albicans

Answer

Histoplasma capsulatum

Answer

Aspergillus fumigatus

Question 3

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.

Accepted Answer

Sporothrix schenckii

Answer

Trichophyton rubrum

Answer

Coccidioides immitis

Answer

Aspergillus flavus

Question 4

A HIV-positive patient presents with chronic diarrhoea for 4 weeks. Stool microscopy shows oval/ elliptical cysts measuring 16 X 20 μm. Diagnosis is?

Accepted Answer

Cystoisospora

Answer

Cryptosporidium

Answer

Cyclospora

Answer

Cryptococcus

Question 5

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?

Accepted Answer

Acanthamoeba

Answer

Pseudomonas aeruginosa

Answer

Herpes simplex virus

Answer

Fusarium

Question 6

Which opportunistic pathogen demonstrates a “crushed ping pong ball appearance” on Gomori methenamine stain?

Accepted Answer

Pneumocystis jirovecii

Answer

Cryptococcus neoformans

Answer

Aspergillus fumigatus

Answer

Histoplasma capsulatum

Question 7

Which antibody is the first to appear and acts fastest during a primary (first) immune response to an infection?

Accepted Answer

IgM

Answer

IgE

Answer

IgA

Answer

IgG

Question 8

Chocolate agar is primarily used for the isolation of which of the following organisms?

Accepted Answer

Haemophilus influenzae

Answer

Listeria monocytogenes

Answer

Streptococcus pneumoniae

Answer

Escherichia coli

Question 9

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?

Accepted Answer

Coxiella burnetii

Answer

Rickettsia typhi

Answer

Orientia tsutsugamushi

Answer

Rickettsia rickettsii

Question 10

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?

Accepted Answer

Sabouraud Dextrose Agar (SDA)

Answer

Chocolate agar

Answer

Thayer-Martin medium

Answer

Loeffler's serum slope

FMGE 2025 — Microbiology