FMGE 2025 - Microbiology Practice Questions

Q: What is the most common causative organism of prosthetic heart valve endocarditis?

S. epidermidis. ***S. epidermidis*** - **S. epidermidis** is the most common single causative organism of prosthetic heart valve (PHV) endocarditis, especially in the **early post-operative period** (within 60 days to 1 year of surgery). - This organism is a **coagulase-negative Staphylococcus** known for its ability to produce a **glycocalyx (biofilm)**, which adheres strongly to foreign materials like prosthetic valves. *S. viridans* - Typically the leading cause of **subacute infectious endocarditis** on native, damaged valves, often following minor **dental procedures**. - While it can cause late PHV endocarditis, it is much less frequent than staphylococcal species, which dominate PHV infections. *S. aureus* - **S. aureus** is the most common cause of **acute native valve endocarditis** and frequently implicated in endocarditis in **IV drug users**. - It is a major cause of PHV endocarditis, especially **late-onset** (more than 1 year post-surgery), but overall, **S. epidermidis** is considered the most common pathogen when considering all PHV infections. *Enterococcus* - **Enterococcus species** (especially *E. faecalis*) account for approximately **5-10% of prosthetic valve endocarditis** cases, particularly in late-onset infections. - While significant, enterococci are far less common than staphylococcal species in PHV endocarditis.

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 patient is positive for HBsAg and anti-HBc IgM. What is the most likely diagnosis?

Acute hepatitis. ***Acute hepatitis***- The simultaneous presence of **HBsAg** (Hepatitis B surface antigen) indicates active infection [1], paired with **anti-HBc IgM** (Immunoglobulin M against the core antigen) [2].- **Anti-HBc IgM** is the serological hallmark of a recent or **acute** Hepatitis B infection, typically detectable for up to 6 months after symptom onset [1].*Chronic hepatitis*- Chronic infection is defined by the persistence of **HBsAg** for **more than six months** [1].- The predominant core antibody present in chronic infection is **anti-HBc IgG**, whereas Anti-HBc IgM becomes undetectable [1].*Recovery phase*- The recovery phase is marked by the presence of **anti-HBs** (Hepatitis B surface antibody), indicating clearance of the virus and protective immunity [1].- During recovery, **HBsAg** is negative, and **anti-HBc IgM** is absent, leaving only **anti-HBc IgG** and anti-HBs [1].*Vaccination*- Successful vaccination only produces immunity against the surface antigen, resulting solely in the presence of **anti-HBs** [1].- Both **HBsAg** and **anti-HBc** (IgM or IgG) are universally negative following vaccination [1].

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 25-year-old female presents with complaints of a yellowish-green, foul-smelling vaginal discharge. She reports painful urination and pain during intercourse & no itching. On speculum examination, her vulva and vaginal walls appear inflamed. What is the causative agent?

Trichomonas vaginalis. ***Trichomonas vaginalis***- The classic clinical presentation of **trichomoniasis** includes a copious, frothy (though not always), **yellowish-green**, and distinctly **foul-smelling** vaginal discharge.- The organism causes inflammation and tissue damage (vaginitis/vulvitis), leading to common symptoms like **dysuria** and **dyspareunia**, often without significant pruritus (itching). *Candida albicans*- This fungal infection typically causes a thick, **white, 'cottage cheese-like'** vaginal discharge.- The hallmark symptom is intense **vulvovaginal pruritus** (itching), which is explicitly reported as absent in this patient. *Gardnerella vaginalis*- This bacterium is the most common cause of **Bacterial Vaginosis (BV)**, which produces a thin, **grayish-white** discharge with a characteristic **fishy odor**.- BV often causes minimal inflammation (no significant vulvitis) and typically does not present with the bright yellowish-green discharge described. *Neisseria gonorrhoeae*- While *N. gonorrhoeae* can cause **cervicitis** and **urethritis** (leading to dysuria), the discharge is usually described as **mucopurulent** rather than copious, foul-smelling, and yellowish-green.- The overall clinical picture (discharge color, odor, and inflammation) points more specifically toward a **prototozoal vaginitis**.

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: A child was brought with c/o multiple lesions. O/E multiple vesicular lesions are seen on the palms, soles, and oral mucosa. Which of the following is the most likely etiological agent causing the disease?

Coxsackie virus. ***Coxsackie virus*** - The clinical presentation of vesicular lesions on the **palms**, **soles**, and **oral mucosa** is the classic triad for **Hand, Foot, and Mouth Disease (HFMD)**. - **Coxsackievirus A16** and **Enterovirus 71** are the most common causes of HFMD, a typically self-limiting illness in young children that spreads via fecal-oral or respiratory routes. *Varicella-zoster virus* - This virus causes **chickenpox**, which presents as a generalized, pruritic, vesicular rash in various stages of healing, typically starting on the **trunk** and spreading centrifugally. - The characteristic distribution of chickenpox is **centripetal**, and prominent involvement of palms and soles is uncommon. *Measles virus* - Measles presents with **maculopapular rash** that begins on the face and spreads cephalocaudally, accompanied by the **3 Cs** (cough, coryza, conjunctivitis) and **Koplik spots** on the buccal mucosa. - The rash is **not vesicular** and typically **spares the palms and soles**, unlike HFMD. *Human-herpesvirus 6* - HHV-6 is the cause of **roseola infantum** (exanthem subitum), characterized by a high fever that resolves, followed by a blanching **maculopapular rash** on the trunk. - Roseola does not cause vesicular lesions and characteristically **spares the palms and soles**.

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*.

Question 1

What is the most common causative organism of prosthetic heart valve endocarditis?

Accepted Answer

S. epidermidis

Answer

S. aureus

Answer

S. viridans

Answer

Enterococcus

Question 2

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 3

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 4

A patient is positive for HBsAg and anti-HBc IgM. What is the most likely diagnosis?

Accepted Answer

Acute hepatitis

Answer

Recovery phase

Answer

Vaccination

Answer

Chronic hepatitis

Question 5

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 6

A 25-year-old female presents with complaints of a yellowish-green, foul-smelling vaginal discharge. She reports painful urination and pain during intercourse & no itching. On speculum examination, her vulva and vaginal walls appear inflamed. What is the causative agent?

Accepted Answer

Trichomonas vaginalis

Answer

Candida albicans

Answer

Neisseria gonorrhoeae

Answer

Gardnerella vaginalis

Question 7

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 8

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 9

A child was brought with c/o multiple lesions. O/E multiple vesicular lesions are seen on the palms, soles, and oral mucosa. Which of the following is the most likely etiological agent causing the disease?

Accepted Answer

Coxsackie virus

Answer

Varicella-zoster virus

Answer

Measles virus

Answer

Human-herpesvirus 6

Question 10

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

FMGE 2025 — Microbiology