Mycology — MCQs

Mycology Indian Medical PG Practice Questions and MCQs

Question 481: Which of the following exists as yeast at 37°C and hyphae at 25°C?

A. Cryptococcus neoformans
B. Histoplasma capsulatum (Correct Answer)
C. Sporothrix globosa
D. Candida albicans

Explanation: ***Histoplasma capsulatum*** - This fungus is **thermally dimorphic**, growing as **yeast** at body temperature (37°C) in the host and as **hyphae (mold)** at cooler temperatures (25°C). - This is the **classic example** taught for thermal dimorphism in medical mycology. - Causes **histoplasmosis**, endemic in Ohio and Mississippi river valleys; presents with pulmonary disease and can disseminate in immunocompromised patients. - Laboratory identification relies on this characteristic temperature-dependent growth pattern. *Sporothrix globosa* - This fungus is also thermally dimorphic with the same pattern: **yeast at 37°C** and **hyphae at 25°C**. - Causes **sporotrichosis** (rose gardener's disease), typically presenting as lymphocutaneous infection. - While medically correct, **Histoplasma capsulatum** is the more commonly referenced example in standard medical textbooks for this specific thermal dimorphism pattern. *Cryptococcus neoformans* - Exists primarily as an **encapsulated yeast** at both temperatures. - Does **not** exhibit thermal dimorphism; remains in yeast form in both host and environment. *Candida albicans* - Exhibits dimorphism but grows as **yeast** at most temperatures and forms **pseudohyphae/germ tubes at 37°C** in response to serum and other factors. - Does not follow the classic thermal dimorphism pattern of yeast at 37°C and mold at 25°C.

Question 482: Beta 1,3 Glucan test is positive in all except?

A. Pneumocystis Jirovecii
B. Candida
C. Mucormycosis (Correct Answer)
D. Aspergillus

Explanation: ***Mucormycosis*** - Fungi causing mucormycosis belong to the order **Mucorales**, which structurally lack **beta-D-glucan** in their cell walls. - Due to the absence of beta-D-glucan, the **beta-1,3-D-glucan assay** will yield a negative result in cases of mucormycosis. *Pneumocystis jirovecii* - This fungus contains significant amounts of **beta-D-glucan** in its cell wall, making the test usually positive during active infection. - A positive **beta-D-glucan test** can be a useful diagnostic marker for **Pneumocystis pneumonia (PCP)**, especially in immunocompromised patients. *Candida* - **Candida species** possess a cell wall rich in **beta-D-glucan**, leading to a positive test result during active infection. - The **beta-D-glucan assay** is a valuable adjunctive test for diagnosing invasive candidiasis. *Aspergillus* - The cell wall of **Aspergillus** contains **beta-D-glucan**, causing the test to be positive in cases of invasive aspergillosis. - A positive **beta-D-glucan test** can aid in the early diagnosis and management of invasive aspergillosis, particularly in high-risk patients.

Question 483: A biopsy of a lung nodule from a patient with a history of bird exposure reveals yeast cells with thick capsules. What is the most likely pathogen?

A. Aspergillus fumigatus
B. Blastomyces dermatitidis
C. Histoplasma capsulatum
D. Cryptococcus neoformans (Correct Answer)

Explanation: ***Cryptococcus neoformans*** - The presence of **yeast cells with thick capsules** is a classic histological finding for *Cryptococcus neoformans*. - While *Cryptococcus* commonly affects immunocompromised individuals, it can also be found in **bird droppings**, particularly from pigeons, making the history of bird exposure relevant. *Aspergillus fumigatus* - *Aspergillus fumigatus* typically presents as **hyphae**, not yeast cells, and would not have a thick capsule. - Infections often manifest as **aspergillomas** (fungus balls) in lung cavities or invasive disease in immunocompromised patients. *Blastomyces dermatitidis* - *Blastomyces dermatitidis* appears as **large, broad-based budding yeast cells** but does not possess a thick capsule. - It is typically found in the **soil**, especially in moist areas, and its association with bird exposure is not as strong as with *Cryptococcus*. *Histoplasma capsulatum* - *Histoplasma capsulatum* is characterized by **small intracellular yeast forms** within macrophages and does not have a thick capsule. - It is strongly associated with **bird and bat droppings** but its microscopic appearance is distinct from that described.

Question 484: A 55-year-old woman presents with persistent cough, fever, and hemoptysis. Sputum shows branching septate hyphae. What is the likely pathogen?

A. Aspergillus fumigatus (Correct Answer)
B. Candida albicans
C. Histoplasma capsulatum
D. Mucor species

Explanation: ***Aspergillus fumigatus*** - The presence of **branching septate hyphae** in sputum, along with symptoms of **persistent cough, fever, and hemoptysis**, is highly characteristic of an *Aspergillus* infection, particularly in immunocompromised patients or those with pre-existing lung conditions. - This fungus often colonizes the respiratory tract and can cause various diseases, including **allergic bronchopulmonary aspergillosis (ABPA)**, **aspergilloma** (fungus ball), or **invasive aspergillosis**. - The hyphae branch at **acute angles (45°)** and are **septate**, which is the key distinguishing feature. *Candida albicans* - While *Candida albicans* is a common fungal pathogen, it typically presents as **yeast** or **pseudohyphae** on microscopy, not branching septate hyphae. - It usually causes **mucocutaneous infections** like thrush or candidemia, with pulmonary involvement being less common and usually presenting differently from the described symptoms. *Histoplasma capsulatum* - *Histoplasma capsulatum* is a **dimorphic fungus** that appears as **small intracellular yeast forms** within macrophages in tissue or sputum, not branching septate hyphae. - It is endemic to certain regions (e.g., Ohio and Mississippi River valleys) and typically causes **pulmonary histoplasmosis**, which can mimic tuberculosis, but microscopic findings differ significantly. *Mucor species* - **Mucor species** are characterized by **broad, ribbon-like, aseptate (non-septate) hyphae** with irregular branching at right angles, which is distinct from the branching septate hyphae described. - These fungi typically cause **mucormycosis** (zygomycosis), an aggressive infection often seen in immunocompromised individuals, especially diabetics with ketoacidosis, and can involve the rhinocerebral region, lungs, or skin.

Question 485: A 65-year-old diabetic man presents with black necrotic tissue on his palate. What is the most likely causative organism?

A. Cryptococcus neoformans
B. Candida albicans
C. Mucor species (Correct Answer)
D. Aspergillus fumigatus

Explanation: ***Mucor species*** - The presence of **black necrotic tissue** on the palate in a diabetic patient is highly suggestive of **mucormycosis**, an aggressive fungal infection caused by *Mucor* species. - **Diabetes mellitus**, particularly with ketoacidosis, is a major risk factor for mucormycosis due to impaired phagocytic function and increased iron availability. *Cryptococcus neoformans* - This fungus is primarily associated with **cryptococcal meningitis** or pneumonia, especially in immunocompromised individuals. - It does not typically cause **black necrotic lesions** on the palate. *Candida albicans* - While *Candida albicans* can cause oral infections (**thrush**), these typically present as white, creamy patches that can be scraped off, not black necrotic tissue. - Oral candidiasis is common in diabetics but does not usually involve tissue necrosis. *Aspergillus fumigatus* - *Aspergillus* species can cause invasive infections, particularly in immunocompromised patients, often affecting the lungs or sinuses. - While it can cause **necrotic lesions**, the characteristic rapid progression and specific presentation in the palate of a diabetic with black necrotic tissue points more strongly towards *Mucor*.

Question 486: Which of the following statements about Candida albicans is most accurate?

A. It is an obligate aerobe.
B. It forms germ tubes at 37°C (Correct Answer)
C. It is the most common cause of nosocomial bloodstream infections.
D. It forms pseudohyphae.

Explanation: ***Correct: It forms germ tubes at 37°C*** - The formation of **germ tubes** when incubated in serum at 37°C for 2-3 hours is a **rapid and classic method** for identifying *Candida albicans* in the laboratory - This is the **most specific and diagnostically important** feature among the options listed - The germ tube test has high sensitivity and specificity for *C. albicans* identification - This dimorphic characteristic is crucial for its pathogenicity and clinical identification *Incorrect: It is an obligate aerobe* - *Candida albicans* is a **facultative anaerobe**, not an obligate aerobe - It can grow both in the presence and absence of oxygen - This metabolic flexibility contributes to its ability to colonize various body sites (mucosal surfaces, bloodstream) and cause diverse infections *Incorrect: It is the most common cause of nosocomial bloodstream infections* - While *Candida* species are significant causes of nosocomial bloodstream infections, **coagulase-negative staphylococci** (e.g., *Staphylococcus epidermidis*) and *Staphylococcus aureus* are more common - *Candida* species typically rank 4th-5th among bloodstream infection pathogens - Among fungi, *C. albicans* is indeed the most common, but the statement is incorrect when considering all pathogens *Incorrect: It forms pseudohyphae* - While this statement is **technically true** (*C. albicans* does form pseudohyphae), it is **not the most accurate** distinguishing answer - **Many other *Candida* species** and fungi can also form pseudohyphae, making this a non-specific feature - The germ tube test is far more **specific and diagnostically useful** for rapid identification of *C. albicans* - Pseudohyphae formation alone cannot reliably differentiate *C. albicans* from other species

Question 487: A patient presents with a severe headache, fever, and neck stiffness. CSF analysis shows increased white blood cells and elevated protein. Which organism is a likely cause?

A. Candida albicans
B. Aspergillus fumigatus
C. Escherichia coli
D. Cryptococcus neoformans (Correct Answer)

Explanation: ***Cryptococcus neoformans*** - This organism is a **fungus** that commonly causes **meningitis**, especially in immunocompromised individuals. - The CSF findings of **increased white blood cells**, **elevated protein**, along with headache, fever, and neck stiffness are characteristic of **meningitis**. *Escherichia coli* - While *E. coli* can cause meningitis, it is more commonly associated with **neonatal meningitis** or **healthcare-associated meningitis**. - It's a bacterium, and while CSF findings would be similar, *Cryptococcus* is a more likely cause for severe, chronic meningitis, particularly if immunodeficiency is present (though not stated, it's a common risk factor for fungal meningitis). *Candida albicans* - *Candida* can cause meningitis, typically in patients with **indwelling catheters** or those who are **severely immunocompromised**. - It often presents with a more subacute to chronic course and might be associated with widespread candidiasis. *Aspergillus fumigatus* - **Aspergillus meningitis** is rare and usually occurs in profoundly **immunocompromised patients** with disseminated aspergillosis, often seen as a complication of central nervous system involvement. - It can be particularly aggressive and carries a high mortality rate.

Question 488: Which species is the most common cause of thrush, vaginitis, and candidemia?

A. Candida albicans (Correct Answer)
B. Cryptococcus neoformans
C. Aspergillus fumigatus
D. Mucor species

Explanation: ***Candida albicans*** - This species is the **most prevalent cause** of opportunistic fungal infections, including oral thrush, vaginal yeast infections, and candidemia. - Its ability to form **germ tubes** and **biofilms** contributes to its pathogenicity and widespread impact on human health. *Cryptococcus neoformans* - Primarily known for causing **meningoencephalitis**, especially in immunocompromised individuals. - Less commonly causes superficial infections like thrush or vaginitis. *Aspergillus fumigatus* - A common cause of **invasive aspergillosis**, often affecting the lungs in immunocompromised patients. - Not typically associated with thrush, vaginitis, or as the leading cause of candidemia. *Mucor species* - Responsible for **mucormycosis**, a rare but aggressive opportunistic infection, particularly in diabetic or immunocompromised patients. - Does not commonly cause thrush, vaginitis, or candidemia.

Question 489: A pathologist finds macrophages laden with yeast-like organisms with clear halos around them in the brain biopsy of a patient with AIDS. Which organism is likely responsible?

A. Histoplasma capsulatum
B. Cryptococcus neoformans (Correct Answer)
C. Aspergillus fumigatus
D. Candida albicans

Explanation: ***Cryptococcus neoformans*** - This description perfectly matches **Cryptococcus neoformans**, which often causes central nervous system infections in **immunocompromised** individuals, particularly those with AIDS. - The "clear halo" refers to the organism's **polysaccharide capsule**, which does not stain with routine hematoxylin and eosin (H&E) stains, making it appear as an unstained area around the yeast cell. *Histoplasma capsulatum* - **Histoplasma capsulatum** is a small, intracellular yeast that is found within macrophages, but it **lacks a prominent capsule** and therefore would not present with a clear halo. - While it can affect the CNS in disseminated disease, the characteristic halo on microscopy is absent. *Aspergillus fumigatus* - **Aspergillus fumigatus** is a filamentous fungus that forms **hyphae**, not yeast-like organisms, and therefore would not appear as described. - It typically causes invasive pulmonary disease and can disseminate to the brain, but the microscopic morphology is distinct (septate hyphae with acute angle branching). *Candida albicans* - **Candida albicans** is a dimorphic fungus that can form yeast cells, **pseudohyphae**, and true hyphae, but it does **not possess a polysaccharide capsule** that creates a clear halo. - While it can cause opportunistic infections in immunocompromised patients, including CNS involvement, its microscopic appearance is different.

Question 490: A 60-year-old diabetic patient with diabetic ketoacidosis develops a black necrotic lesion on the palate. Culture reveals a mold with nonseptate hyphae. What is the likely organism?

A. Aspergillus fumigatus
B. Candida albicans
C. Rhizopus oryzae (Correct Answer)
D. Cryptococcus neoformans

Explanation: ***Rhizopus oryzae*** - The presentation of a **black necrotic lesion** on the palate in a **diabetic ketoacidosis** patient is classic for **mucormycosis**, often caused by *Rhizopus*. - The finding of **nonseptate hyphae** on culture is a key mycological characteristic of *Rhizopus* and other Mucorales. *Aspergillus fumigatus* - While *Aspergillus* can cause invasive fungal infections in immunocompromised patients, it typically presents with **septate hyphae** with acute-angle branching, unlike the nonseptate hyphae described. - Infections commonly involve the lungs or sinuses but less frequently present as a black necrotic palatal lesion in this context. *Candida albicans* - *Candida* is a yeast that can form pseudohyphae and true septate hyphae in tissue, but the culture would show budding yeast cells, not a **mold with nonseptate hyphae**. - It causes oral thrush and other candidiasis forms but would not cause a rapidly progressive, black necrotic lesion in the palate in this specific manner. *Cryptococcus neoformans* - *Cryptococcus* is an encapsulated yeast primarily known for causing **meningitis** in immunocompromised individuals. - It does not form hyphae and is not associated with black necrotic palatal lesions.

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Classification of Fungi

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Superficial Mycoses

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Dermatophytes

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Subcutaneous Mycoses

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Candidiasis

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Aspergillosis

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Cryptococcosis

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Zygomycosis

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Endemic Mycoses

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Opportunistic Fungal Infections

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Antifungal Agents

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Laboratory Diagnosis of Fungal Infections

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Mycology — MCQs

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