Mycology — MCQs

Mycology Indian Medical PG Practice Questions and MCQs

Question 391: Histoplasma capsulatum is a:

A. Protozoa
B. Dimorphic fungus (Correct Answer)
C. Amphotericin resistant
D. Capsulated

Explanation: **Explanation:** *Histoplasma capsulatum* is the causative agent of Histoplasmosis (Darling’s disease). The correct answer is **Dimorphic fungus** because it exhibits two distinct morphological forms depending on the environmental temperature: 1. **At 25°C (Saprophytic phase):** It grows as a mold with hyphae, producing characteristic large, thick-walled, spiked **tuberculate macroconidia**. 2. **At 37°C (Parasitic phase):** It grows as small, oval budding yeast cells within the host’s macrophages. **Analysis of Incorrect Options:** * **A. Protozoa:** Despite its name and its intracellular nature (resembling *Leishmania*), it is a fungus, not a parasite. * **C. Amphotericin resistant:** This is incorrect. Amphotericin B is the gold standard treatment for severe or disseminated Histoplasmosis. * **D. Capsulated:** This is a **classic distractor**. Despite the name *capsulatum*, this fungus is **not** encapsulated. The "halo" seen around the yeast cells in tissue sections is actually a staining artifact caused by the shrinkage of the cytoplasm away from the cell wall. **High-Yield Clinical Pearls for NEET-PG:** * **Habitat:** Found in soil enriched with **bird or bat droppings** (guano); common among cave explorers (Spelunker’s lung). * **Pathogenesis:** It is an **intracellular** pathogen that survives inside **macrophages**. * **Clinical Presentation:** Can mimic Tuberculosis (hilar lymphadenopathy and lung calcifications). * **Diagnosis:** Best diagnosed via fungal culture (SDA) or by observing intracellular yeasts in peripheral blood smears or bone marrow aspirates.

Question 392: Aseptate hyphae are not seen in which of the following?

A. Rhizopus
B. Mucor
C. Aspergillus (Correct Answer)
D. None of the above

Explanation: **Explanation:** The fundamental distinction in medical mycology is between **septate** and **aseptate (coenocytic)** hyphae. **1. Why Aspergillus is the correct answer:** *Aspergillus* species belong to the phylum Ascomycota. They are characterized by **septate hyphae** (possessing cross-walls) that typically exhibit **dichotomous branching at acute angles (roughly 45°)**. Because they possess septa, they are the correct choice for a fungus that does *not* have aseptate hyphae. **2. Why the other options are incorrect:** * **Rhizopus and Mucor:** Both belong to the order **Mucorales** (formerly Zygomycota). These fungi are classically described as having **broad, ribbon-like, aseptate (or sparsely septate) hyphae** that branch at wide angles (90°/obtuse). Therefore, they *do* exhibit aseptate hyphae. **3. NEET-PG High-Yield Clinical Pearls:** * **Mucormycosis (Rhizopus/Mucor):** Associated with uncontrolled Diabetes Mellitus (Ketoacidosis) and post-COVID complications. Look for "broad, ribbon-like, 90° branching" in histopathology. * **Aspergillosis:** Look for "slender, septate, 45° branching" in histopathology. It can present as an Aspergilloma ("fungus ball" in pre-existing TB cavities) showing the **Monod sign** (air crescent) on X-ray. * **Culture:** Both grow on Sabouraud Dextrose Agar (SDA). *Rhizopus* is a "lid-lifter" due to its rapid growth. * **Stains:** Silver stains (GMS) and PAS are used to visualize these structures in tissue sections. **Summary Table for Quick Revision:** | Feature | Mucorales (Rhizopus/Mucor) | Aspergillus | | :--- | :--- | :--- | | **Septation** | Aseptate / Coenocytic | Septate | | **Branching Angle** | Wide / Right angle (90°) | Acute angle (45°) | | **Hyphae Width** | Broad, ribbon-like | Thin, uniform |

Question 393: What is the preferred culture medium for isolating the yeast form of dimorphic fungi?

A. Brain Heart Infusion (BHI) agar
B. Sabouraud Dextrose Agar (SDA)
C. SDA with antibiotics, incubated at 37°C (Correct Answer)
D. Any suitable medium incubated at 35-37°C

Explanation: ### Explanation **Concept of Fungal Dimorphism:** Dimorphic fungi (e.g., *Histoplasma*, *Blastomyces*, *Coccidioides*, *Sporothrix*) exist in two forms: a **mold** form in the environment (25-30°C) and a **yeast** form in host tissues or enriched media (37°C). To isolate the yeast form in a laboratory setting, two conditions must be met: an enriched medium and an incubation temperature of 37°C. **Why Option C is Correct:** **Sabouraud Dextrose Agar (SDA)** is the standard medium for fungal growth. However, when isolating the yeast form from clinical samples (like sputum or pus), the addition of **antibiotics** (e.g., chloramphenicol and gentamicin) is crucial to inhibit commensal bacterial overgrowth. Incubation at **37°C** provides the thermal stimulus required for the fungus to express its yeast morphology. **Analysis of Incorrect Options:** * **Option A (BHI Agar):** While BHI is a highly enriched medium often used for fastidious fungi, SDA remains the primary diagnostic standard. Without the specification of temperature and antibiotics, it is less precise than Option C. * **Option B (SDA):** Plain SDA at room temperature (25°C) will grow the mold/mycelial phase, not the yeast phase. * **Option D:** "Any suitable medium" is too vague. Dimorphic fungi are fastidious; they require specific nutrients and controlled temperatures to undergo phase conversion. **High-Yield NEET-PG Pearls:** * **Mnemonic for Dimorphic Fungi:** "**B**ody **H**eat **P**robably **C**hanges **S**hape" (**B**lastomyces, **H**istoplasma, **P**aracoccidioides, **C**occidioides, **S**porothrix). * **Exception:** *Coccidioides immitis* forms **spherules** (not typical yeast) in the body at 37°C. * **Culture Duration:** Fungal cultures are typically incubated for up to 4 weeks before being declared negative. * **Safety:** The mold form (25°C) is highly infectious; laboratory-acquired infections occur via inhalation of spores (conidia).

Question 394: Latex agglutination test is done for which of the following microorganisms?

A. Cryptococcus neoformans (Correct Answer)
B. Candida
C. Coccidiodes
D. Paracoccidiodes

Explanation: **Explanation:** The **Latex Agglutination Test (LAT)** is the gold standard rapid diagnostic method for detecting the **capsular polysaccharide antigen (Glucuronoxylomannan)** of *Cryptococcus neoformans*. 1. **Why Cryptococcus is correct:** *Cryptococcus* is unique among pathogenic fungi due to its prominent polysaccharide capsule. The LAT uses latex beads coated with specific anti-cryptococcal antibodies. When mixed with a patient’s serum or Cerebrospinal Fluid (CSF), these beads agglutinate in the presence of the capsular antigen. It is highly sensitive (>90%) and specific, often replacing India Ink (which has lower sensitivity) for the diagnosis of Cryptococcal meningitis. 2. **Why other options are incorrect:** * **Candida:** Diagnosis primarily relies on microscopy, culture, or the detection of **(1,3)-beta-D-glucan** or mannan antigens. * **Coccidioides & Paracoccidioides:** These are dimorphic fungi. Diagnosis is typically established via histopathology (showing spherules or "pilot wheel" yeast forms), culture, or skin tests (Coccidioidin). While serology (ELISA/Immunodiffusion) is used, LAT is not the primary diagnostic modality. **High-Yield Clinical Pearls for NEET-PG:** * **Specimen of choice:** CSF is preferred for meningitis; serum can be used for disseminated disease. * **Prognostic Value:** The **titer** of the antigen in LAT correlates with the fungal burden and treatment response. * **False Positives:** Can occur due to **Rheumatoid Factor (RF)** or infection with *Trichosporon beigelii*. * **India Ink:** Shows a "negative staining" effect where the capsule appears as a clear halo against a dark background.

Question 395: Most fulminant fungal meningitis is caused by which organism?

A. Coccidioides
B. Histoplasma
C. Cryptococcus (Correct Answer)
D. Mucormycosis

Explanation: **Explanation:** **Cryptococcus neoformans** is the most common cause of fungal meningitis worldwide and is characterized by its ability to cause a rapid, life-threatening (fulminant) infection, particularly in immunocompromised individuals (e.g., HIV/AIDS patients with CD4 counts <100 cells/µL). The organism's thick **polysaccharide capsule** allows it to evade the immune system, leading to high intracranial pressure and significant neurological morbidity. **Why the other options are incorrect:** * **Coccidioides:** While *Coccidioides immitis* can cause chronic meningitis (especially in the Southwestern US), it typically follows a more indolent, long-term course rather than a fulminant presentation. * **Histoplasma:** *Histoplasma capsulatum* primarily causes pulmonary disease. While disseminated histoplasmosis can involve the CNS, it is a rare manifestation and usually presents as a chronic process. * **Mucormycosis:** While highly fulminant, *Mucor* typically causes **Rhinocerebral** disease (invading the palate, orbit, and brain parenchyma directly) rather than a primary leptomeningitis. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** India Ink preparation shows a "halo" (capsule); **Cryptococcal Antigen (CrAg)** test is the most sensitive rapid test. * **Culture:** Grows on **Sabouraud Dextrose Agar (SDA)**; produces brown/black colonies on **Niger Seed/Bird Seed Agar** (due to urease/phenoloxidase activity). * **Pathology:** "Soap bubble" appearance in the basal ganglia on MRI. * **Treatment:** Induction with Amphotericin B + Flucytosine, followed by Fluconazole for maintenance.

Question 396: What is an ascospore?

A. An asexual spore
B. A sexual spore (Correct Answer)
C. Conidia
D. None of the above

Explanation: **Explanation:** In medical mycology, fungi are classified based on their mode of reproduction. Spores are the primary means of reproduction and are categorized into sexual and asexual types. **Why the correct answer is right:** **Ascospores** are **sexual spores** produced by fungi belonging to the phylum **Ascomycota** (the largest phylum of fungi). They are formed following the fusion of two nuclei (karyogamy) and subsequent meiosis. These spores are typically contained within a specialized sac-like structure called an **ascus**. Each ascus usually contains eight ascospores. **Why the incorrect options are wrong:** * **Option A (Asexual spore):** Asexual spores are formed by mitosis without the fusion of gametes. Common examples include sporangiospores (seen in Zygomycetes like *Rhizopus*) and various types of conidia. * **Option C (Conidia):** Conidia are specifically **asexual** spores that are produced at the tips or sides of specialized hyphae called conidiophores. They are not enclosed in a sac (unlike sporangiospores). **NEET-PG High-Yield Pearls:** 1. **Sexual Spores (Mnemonic: ZAB):** **Z**ygospores, **A**scospores, and **B**asidiospores. 2. **Asexual Spores:** These include Conidia (Microconidia/Macroconidia), Sporangiospores, Arthrospores (fragmentation), and Chlamydospores (resting spores). 3. **Clinical Relevance:** Most human fungal pathogens are "Fungi Imperfecti" (Deuteromycetes), meaning their sexual stage (teleomorph) is either unknown or rarely seen in clinical settings; they primarily reproduce via asexual spores (anamorph). 4. **Ascomycota Examples:** *Histoplasma capsulatum*, *Blastomyces dermatitidis*, and *Aspergillus* species are medically important fungi that belong to this phylum.

Question 397: All of the following statements regarding fungi are true EXCEPT:

A. Yeasts are unicellular fungi.
B. Candida species can form pseudomycelia.
C. Dermatophytes are typically filamentous fungi.
D. Dimorphic fungi behave as molds in cultures incubated at 37°C. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**. This statement is false because dimorphic fungi follow the rule: **"Mold in the Cold, Yeast in the Beast."** At room temperature (25°C–30°C) or in nature, they grow as filamentous **molds**. However, at body temperature (37°C) or on enriched media in an incubator, they transition into **yeasts** (unicellular forms). This morphological shift is a key virulence factor for pathogens like *Histoplasma*, *Blastomyces*, and *Coccidioides*. **Analysis of other options:** * **Option A:** True. Yeasts are defined as unicellular fungi that typically reproduce by budding (e.g., *Saccharomyces*). * **Option B:** True. *Candida albicans* is unique because it can exist as yeast, form **pseudohyphae** (elongated budding cells that remain attached), and produce true hyphae (germ tubes). * **Option C:** True. Dermatophytes (e.g., *Trichophyton*, *Microsporum*) are monomorphic filamentous fungi (molds) that invade keratinized tissues. **High-Yield NEET-PG Pearls:** * **Exceptions to Dimorphism:** *Candida* is often called "reverse dimorphic" because it forms hyphae/germ tubes at 37°C (in tissues) and yeast at lower temperatures. * **Coccidioides:** While dimorphic, it forms **spherules** containing endospores in the tissue (37°C) rather than simple yeast cells. * **Culture:** Sabouraud Dextrose Agar (SDA) is the standard medium for fungal growth; actidione (cycloheximide) is added to inhibit saprophytic fungi.

Question 398: What is the investigation of choice for cryptococcosis?

A. Culture
B. India ink (Correct Answer)
C. PCR
D. Acid fast stain

Explanation: **Explanation:** **Cryptococcosis**, caused primarily by *Cryptococcus neoformans*, is a significant opportunistic fungal infection, especially in HIV/AIDS patients. **Why India Ink is the Correct Answer:** The hallmark of *Cryptococcus* is its **thick polysaccharide capsule**. India ink (a negative stain) is the classic investigation of choice for rapid diagnosis, particularly in cerebrospinal fluid (CSF) samples. The ink particles are excluded by the capsule, creating a clear, translucent **"halo"** against a dark background under the microscope. This provides an immediate, presumptive diagnosis of Cryptococcal meningitis. **Analysis of Incorrect Options:** * **A. Culture:** While fungal culture (on Sabouraud Dextrose Agar) is the "Gold Standard" for definitive diagnosis, it takes 48–72 hours to show creamy, mucoid colonies. In an acute clinical setting, India ink is preferred for its speed. * **C. PCR:** Though highly sensitive, PCR is not yet the routine investigation of choice in standard clinical protocols due to cost and availability compared to microscopy and antigen testing. * **D. Acid Fast Stain:** This is used for Mycobacteria (*M. tuberculosis*). While a modified Kinyoun stain can identify *Nocardia*, it is not used for *Cryptococcus*. **High-Yield NEET-PG Pearls:** * **Most Sensitive Test:** The **Cryptococcal Antigen (CrAg)** test via Lateral Flow Assay (LFA) or Latex Agglutination is now considered more sensitive (>95%) than India ink (~50-80%). If "Antigen detection" were an option, it would be the superior diagnostic choice. * **Other Stains:** In tissue histopathology, use **Mucicarmine** (stains capsule red), **Masson-Fontana** (stains melanin in cell wall), or PAS/GMS. * **Biochemical Key:** *Cryptococcus* is **Urease positive**. * **Clinical Sign:** On lumbar puncture, patients typically show **markedly elevated opening pressure**.

Question 399: What is the most common cause of Tinea capitis?

A. M. canis (Correct Answer)
B. Epidermophyton floccosum
C. T. tonsurans
D. T. concentricum

Explanation: **Explanation:** **Tinea capitis** is a fungal infection of the scalp and hair shafts, primarily affecting children. The correct answer is **Microsporum canis**, which is the most common cause of Tinea capitis worldwide, particularly in the Mediterranean and parts of Asia. It is a zoophilic fungus often transmitted to humans through contact with infected cats or dogs. **Analysis of Options:** * **M. canis (Correct):** It is the leading cause of inflammatory Tinea capitis. It typically causes an **ectothrix** infection (spores on the outside of the hair shaft) and exhibits a bright green fluorescence under **Wood’s lamp**. * **Epidermophyton floccosum:** This organism primarily affects the skin and nails (Tinea cruris and Tinea pedis). It is unique among dermatophytes because it **never infects the hair**. * **T. tonsurans:** While it is the most common cause of Tinea capitis in the **United States** and parts of the UK, it is not the most common cause globally. It causes an **endothrix** infection and is the primary agent behind "Black Dot" Tinea capitis. * **T. concentricum:** This is the specific causative agent of **Tinea imbricata**, characterized by distinctive concentric rings of scales, found mostly in the South Pacific and South America. **High-Yield Clinical Pearls for NEET-PG:** 1. **Wood’s Lamp:** *Microsporum* species (M. canis, M. audouinii) fluoresce green; *Trichophyton* species (except T. schoenleinii) do not. 2. **Ectothrix vs. Endothrix:** *Microsporum* is typically Ectothrix; *Trichophyton* is typically Endothrix. 3. **Favus:** A severe form of Tinea capitis characterized by scutula (crusts) and permanent alopecia, caused by **T. schoenleinii**. 4. **Treatment:** Oral **Griseofulvin** remains the gold standard for Tinea capitis, as topical agents cannot penetrate the hair follicle.

Question 400: Which of the following is NOT a characteristic of Cryptococcus neoformans?

A. A non-encapsulated fungus that infects humans (Correct Answer)
B. Causes meningitis
C. Melanin production is related to virulence
D. A basidiomycetes yeast

Explanation: **Explanation:** The correct answer is **A**. *Cryptococcus neoformans* is defined by its **prominent polysaccharide capsule**, which is its most important virulence factor. This capsule is composed primarily of glucuronoxylomannan (GXM), which inhibits phagocytosis and allows the yeast to evade the host immune system. In clinical practice, this capsule is visualized using **India Ink preparation**, appearing as a clear halo against a dark background. **Analysis of other options:** * **Option B:** It is the most common cause of fungal meningitis, particularly in immunocompromised individuals (HIV/AIDS patients with CD4 counts <100 cells/μL). * **Option C:** *C. neoformans* produces the enzyme **phenoloxidase (laccase)**, which converts phenolic compounds into **melanin**. Melanin protects the fungus from oxidative stress and host immune responses, making it a key virulence factor. This property is used for identification on **Niger Seed (Birdseed) Agar**, where colonies appear brown/black. * **Option D:** Taxonomically, *Cryptococcus* is a **Basidiomycete** yeast. Its sexual state (teleomorph) is known as *Filobasidiella neoformans*. **High-Yield NEET-PG Pearls:** * **Source:** Found in soil enriched with **pigeon droppings**. * **Diagnosis:** **Latex Agglutination Test** (detects capsular antigen) is more sensitive than India Ink. * **Culture:** Grows on Sabouraud Dextrose Agar (SDA) as mucoid, cream-colored colonies. It is **Urease positive**. * **Treatment:** Induction therapy with Amphotericin B + Flucytosine, followed by Fluconazole.

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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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