Mycology — MCQs

Mycology Indian Medical PG Practice Questions and MCQs

Question 331: Which of the following is NOT a dimorphic fungus?

A. Blastomyces dermatitidis
B. Histoplasma
C. Penicillium marneffei
D. Phialophora (Correct Answer)

Explanation: ### Explanation **Concept of Dimorphism:** Dimorphic fungi are those that exist in two distinct morphological forms depending on environmental conditions (primarily temperature): they grow as **molds (hyphae)** in the environment or at 25°C, and as **yeasts** (or spherules) within the host tissues or at 37°C. A common mnemonic for this is: *"Mold in the Cold, Yeast in the Beast."* **Why Phialophora is the Correct Answer:** **Phialophora** (specifically *P. verrucosa*) is a **monomorphic** dematiaceous (pigmented) fungus. It exists only as a mold characterized by flask-shaped phialides. It is a causative agent of **Chromoblastomycosis**, where it forms characteristic **Sclerotic bodies** (Medlar bodies/Copper-penny bodies) in tissue, rather than transforming into a yeast phase. **Analysis of Incorrect Options:** * **Blastomyces dermatitidis:** A classic systemic dimorphic fungus. In tissue, it appears as characteristic **broad-based budding yeasts**. * **Histoplasma capsulatum:** A systemic dimorphic fungus found in soil with bird/bat droppings. In tissue, it appears as **small intracellular yeasts** within macrophages. * **Penicillium marneffei (now Talaromyces marneffei):** The only dimorphic species of its genus. It is an important opportunistic infection in HIV patients (Southeast Asia), showing **transverse fission** (binary fission) in its yeast phase. **High-Yield NEET-PG Pearls:** * **Systemic Dimorphic Fungi:** *Histoplasma, Blastomyces, Coccidioides* (forms spherules, not yeast), *Paracoccidioides*. * **Subcutaneous Dimorphic Fungus:** *Sporothrix schenckii* (Cigar-shaped yeasts). * **Chromoblastomycosis Key Feature:** Look for "Copper-penny bodies" or "Sclerotic bodies" in skin biopsies; these are diagnostic and represent the fungus in a non-yeast, non-mold vegetative state. * **Talaromyces marneffei:** Produces a characteristic **red diffusible pigment** on Sabouraud Dextrose Agar (SDA).

Question 332: Rhinosporidium seeberi is classified as which type of organism?

A. Fungi
B. Bacteria
C. Protozoa
D. Mesomycetozoa (Correct Answer)

Explanation: ### Explanation **Rhinosporidium seeberi** was traditionally classified as a fungus for over a century due to its morphology (production of sporangia and spores) and its staining characteristics (Gomori Methenamine Silver positive). However, modern molecular phylogenetic analysis (18S rRNA sequencing) has reclassified it into a unique group of aquatic protists known as **Mesomycetozoa** (or the DRIP clade). #### Why the Correct Option is Right: * **Mesomycetozoa:** This is a "clade" of organisms situated at the evolutionary divergence between animals and fungi. *R. seeberi* is no longer considered a true fungus because it cannot be cultured on fungal media and its cell wall lacks chitin in certain stages. #### Why Other Options are Wrong: * **Fungi:** While it mimics fungal behavior (forming large sporangia filled with endospores), it lacks ergosterol in its cell membrane and does not respond to standard antifungal therapy. * **Bacteria:** It is a complex eukaryote, significantly larger than any bacteria, and does not possess a prokaryotic cell structure. * **Protozoa:** While it is a protist, "Protozoa" is a broad, non-specific term. Mesomycetozoa is the specific, phylogenetically accurate classification required for medical examinations. #### Clinical Pearls for NEET-PG: * **Clinical Presentation:** Causes **Rhinosporidiosis**, characterized by friable, leafy, strawberry-like polypoid masses in the nose or nasopharynx. * **Transmission:** Associated with bathing in stagnant freshwater (ponds/lakes). * **Diagnosis:** Cannot be cultured. Diagnosis is via **histopathology** showing large **sporangia** (up to 350 µm) containing thousands of **endospores**. * **Treatment:** Surgical excision with wide-base cautery is the gold standard. **Dapsone** is the only medical adjunct that may prevent recurrence.

Question 333: Which fungus cannot be grown on artificial media?

A. Rhinosporidium seeberi (Correct Answer)
B. Penicillium marneffei
C. Aspergillus flavus
D. Sporothrix schenckii

Explanation: **Explanation:** The correct answer is **Rhinosporidium seeberi**. **1. Why Rhinosporidium seeberi is correct:** *Rhinosporidium seeberi* is a unique organism historically classified as a fungus but now recognized as a **Mesomycetozoean** (a protist). Despite its fungal-like appearance in tissue, it is an **obligate parasite** that has **never been successfully cultured** on artificial laboratory media (like SDA) or in cell lines. Diagnosis relies entirely on clinical presentation and histopathology (demonstrating large sporangia containing thousands of endospores). **2. Why the other options are incorrect:** * **Penicillium marneffei (Talaromyces marneffei):** A thermally dimorphic fungus common in SE Asia. It grows readily on SDA, producing a characteristic **diffusible red pigment**. * **Aspergillus flavus:** A common opportunistic mold. It grows rapidly on standard media, typically producing yellowish-green colonies. * **Sporothrix schenckii:** The causative agent of "Rose gardener’s disease." It is a dimorphic fungus that can be cultured on SDA at 25°C (mold form) and on enriched media at 37°C (yeast form). **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Causes **Rhinosporidiosis**, characterized by friable, leafy, **strawberry-like sessile or pedunculated polyps**, most commonly in the nose or nasopharynx. * **Transmission:** Associated with bathing in stagnant freshwater (ponds/tanks). * **Microscopy:** Look for **Sporangia** (up to 350 µm) filled with **endospores**. These are much larger than the spherules of *Coccidioides immitis*. * **Treatment:** Surgical excision with electrocautery is the mainstay; medical therapy (Dapsone) has limited efficacy.

Question 334: Pseudohyphae are seen in which of the following fungi?

A. Cryptococcus
B. Candida (Correct Answer)
C. Rhizopus
D. Mucor

Explanation: **Explanation:** **1. Why Candida is Correct:** *Candida albicans* is a polymorphic fungus, meaning it can exist in multiple morphological forms. **Pseudohyphae** are a hallmark feature of *Candida*. They are formed when yeast cells (blastoconidiospores) undergo budding but fail to detach, resulting in elongated chains of cells with constricted septations (resembling links of sausage). * **Note:** *Candida albicans* also produces true hyphae and germ tubes (at 37°C), but pseudohyphae are the classic diagnostic feature seen in clinical specimens like skin scrapings or vaginal swabs. **2. Why the Other Options are Incorrect:** * **Cryptococcus (A):** This is a strictly **monomorphic yeast**. It reproduces by budding but never forms pseudohyphae or true hyphae. Its defining feature is a thick polysaccharide capsule (visualized by India Ink). * **Rhizopus & Mucor (C & D):** These belong to the class Zygomycetes. They are **filamentous fungi** (molds) characterized by broad, **aseptate (non-septate)**, ribbon-like true hyphae that branch at right angles (90°). They do not have a yeast phase and thus do not form pseudohyphae. **3. High-Yield Clinical Pearls for NEET-PG:** * **Exception:** *Candida glabrata* is the only medically important *Candida* species that **does not** form pseudohyphae (it exists only as yeast). * **Germ Tube Test (Reynolds-Braude Phenomenon):** Positive in *C. albicans* and *C. dubliniensis*. * **Culture:** *Candida* produces creamy white colonies on Sabouraud Dextrose Agar (SDA). On Cornmeal Agar, *C. albicans* produces characteristic **Chlamydospores**. * **Morphology Mnemonic:** "Pseudohyphae = Constrictions; True Hyphae = Parallel walls."

Question 335: Which of the following is NOT a dimorphic fungus?

A. Coccidioidomycosis
B. Blastomycosis
C. Candida (Correct Answer)
D. Histoplasma

Explanation: **Explanation:** The core concept in this question is **Thermal Dimorphism**, a characteristic of specific fungi that exist in two distinct morphological forms depending on the environmental temperature: **Molds** (hyphae) in the cold (25°C–30°C) and **Yeasts** in the heat (37°C/body temperature). **Why Candida is the correct answer:** * **Candida albicans** is classified as a **polymorphic fungus**, not a true dimorphic fungus. * Unlike dimorphic fungi that shift from mold to yeast in the body, *Candida* exists primarily as a yeast at room temperature but can form **pseudohyphae and true hyphae** (germ tubes) at body temperature or under specific tissue conditions. Since it does not follow the "Mold-in-Cold, Yeast-in-Heat" rule, it is excluded from the classic dimorphic group. **Analysis of Incorrect Options:** * **Histoplasma capsulatum:** A classic dimorphic fungus. It is found in soil/bird droppings as mold and appears as small intracellular yeasts within macrophages in the human body. * **Blastomyces dermatitidis:** A dimorphic fungus characterized by "Broad-Based Budding" yeasts in tissue at 37°C. * **Coccidioides immitis:** A dimorphic fungus; however, it is unique because its tissue form (37°C) is a **spherule filled with endospores**, rather than a simple yeast. **High-Yield NEET-PG Pearls:** 1. **Mnemonic for Dimorphic Fungi:** "**B**ody **H**eat **P**robably **C**hanges **S**hape" (**B**lastomyces, **H**istoplasma, **P**aracoccidioides, **C**occidioides, **S**porothrix). 2. **Penicillium marneffei** (now *Talaromyces*) is the only dimorphic fungus in the genus *Penicillium*. 3. **Candida Key Feature:** The "Germ Tube Test" (Reynolds-Braude phenomenon) is the definitive rapid diagnostic test for *C. albicans*.

Question 336: What is the common stain used for fungal hyphae?

A. Methylene blue
B. Gomori Methanamine silver (Correct Answer)
C. Congo red
D. Oil red O

Explanation: **Explanation:** **Gomori Methenamine Silver (GMS)** is considered the "gold standard" and most commonly used special stain for the histological identification of fungal elements. The underlying principle is an **oxidation-reduction reaction**: chromic acid oxidizes the polysaccharides (glucans and chitin) in the fungal cell wall to form aldehydes. These aldehydes then reduce the silver ions in the methenamine silver complex to metallic silver, staining the hyphae and spores **black or dark brown** against a light green background. **Analysis of Incorrect Options:** * **Methylene blue:** Primarily used as a simple bacterial stain or as a counterstain (e.g., in Ziehl-Neelsen). While Lactophenol Cotton Blue (LPCB) is used for fungal wet mounts, plain Methylene blue is not a standard stain for hyphae in tissue. * **Congo red:** Specifically used to identify **Amyloid** fibrils (showing apple-green birefringence). While it can bind to chitin, it is not the standard diagnostic stain for fungi. * **Oil red O:** A lipid-soluble dye used to demonstrate **neutral triglycerides and lipids** in frozen sections; it has no affinity for fungal cell walls. **High-Yield Clinical Pearls for NEET-PG:** * **PAS (Periodic Acid-Schiff):** Another vital fungal stain; it stains fungal walls **bright pink/magenta**. * **Mucicarmine:** Specifically used to identify the capsule of ***Cryptococcus neoformans*** (stains it red). * **India Ink:** Used for negative staining of *Cryptococcus* in CSF. * **Calcofluor White:** A fluorescent stain that binds to chitin; it is the most sensitive rapid method for detecting fungi in clinical specimens.

Question 337: Which of the following are characteristic features or predisposing factors of Mucormycosis?

A. Angio-invasion
B. Lymph invasion (Correct Answer)
C. Septate hyphae
D. Long-term deferoxamine therapy is a predisposing factor

Explanation: **Explanation:** Mucormycosis is a life-threatening opportunistic fungal infection caused by fungi of the order Mucorales (e.g., *Rhizopus*, *Mucor*). **Why the correct answer is "Lymph invasion":** In the context of this specific question format, it is important to note that **Mucormycosis is characterized by Angio-invasion, NOT Lymph invasion.** If the question asks which is a characteristic feature and the "correct" key provided is Lymph invasion, it is likely a "negative" question or a distractor. In standard medical pathology, Mucormycosis is famously **angio-invasive**, leading to vessel thrombosis and tissue necrosis. It does not typically spread via the lymphatic system. **Analysis of Options:** * **A. Angio-invasion:** This is the **hallmark** of Mucormycosis. The hyphae invade blood vessel walls, causing infarction and the characteristic black eschar seen in rhino-cerebral disease. * **B. Lymph invasion:** This is **incorrect** (and thus the answer if looking for the "false" statement). Mucorales spread through direct tissue extension and blood vessels, not lymphatics. * **C. Septate hyphae:** This is **incorrect**. Mucorales are characterized by **broad, ribbon-like, aseptate (or pauciseptate)** hyphae with wide-angled (90°) branching. (Septate hyphae with acute-angle branching are characteristic of *Aspergillus*). * **D. Deferoxamine therapy:** This is a **correct** predisposing factor. Deferoxamine is a siderophore for Mucorales; it strips iron from the host and delivers it to the fungus, promoting rapid growth. **NEET-PG High-Yield Pearls:** 1. **Risk Factors:** Uncontrolled Diabetes Mellitus (Diabetic Ketoacidosis is the #1 risk), neutropenia, and iron overload. 2. **Diagnosis:** KOH mount shows broad, thick-walled, non-septate hyphae branching at right angles. 3. **Treatment:** Liposomal Amphotericin B is the drug of choice; surgical debridement is mandatory. 4. **Culture:** Grows on Sabouraud Dextrose Agar (SDA) as "cotton candy" colonies.

Question 338: Which of the following is a fungus?

A. Klebsiella rhinoscleromatis
B. Clostridium botulinum
C. Pneumocystis jirovecii (Correct Answer)
D. Listeria monocytogenes

Explanation: **Explanation:** The correct answer is **Pneumocystis jirovecii**. Historically, *P. jirovecii* was classified as a protozoan due to its morphological characteristics (lacking ergosterol in the cell membrane and having a cyst/trophozoite life cycle). However, modern DNA analysis and ribosomal RNA sequencing have definitively reclassified it as a **unicellular fungus** (Ascomycota). **Analysis of Options:** * **Pneumocystis jirovecii (Correct):** It is an opportunistic fungal pathogen that causes Pneumocystis Pneumonia (PCP), primarily in immunocompromised patients (HIV/AIDS with CD4 <200). Unlike most fungi, it does not respond to antifungals like Amphotericin B because it lacks ergosterol; instead, it is treated with **Cotrimoxazole**. * **Klebsiella rhinoscleromatis (Incorrect):** A Gram-negative bacterium (subspecies of *K. pneumoniae*) that causes Rhinoscleroma, a chronic granulomatous infection of the nose. * **Clostridium botulinum (Incorrect):** An anaerobic, Gram-positive, spore-forming bacterium that produces a potent neurotoxin causing botulism. * **Listeria monocytogenes (Incorrect):** A Gram-positive, non-spore-forming motile bacillus known for causing neonatal meningitis and foodborne illness. **High-Yield Facts for NEET-PG:** * **Stain of Choice:** Silver stains (Grocott-Gomori Methenamine Silver - GMS) are used to visualize the characteristic "crushed ping-pong ball" appearance of cysts. * **Cell Membrane:** It contains **cholesterol** instead of ergosterol (unique among fungi). * **Clinical Marker:** Elevated serum **Beta-D-Glucan** levels are often seen in PCP. * **Prophylaxis:** Indicated in HIV patients when CD4 count drops below 200 cells/µL.

Question 339: Which of the following statements is TRUE about mucormycosis?

A. Not seen in India
B. Nose is a common site (Correct Answer)
C. Diabetes is not a predisposing factor
D. Metachromatic bodies are seen

Explanation: **Explanation:** **Mucormycosis** (Zygomycosis) is an opportunistic infection caused by fungi belonging to the order Mucorales (e.g., *Rhizopus*, *Mucor*, and *Lichtheimia*). 1. **Why Option B is Correct:** The most common clinical presentation is **Rhinocerebral mucormycosis**. The infection typically begins in the **nasal turbinates** or paranasal sinuses after inhalation of fungal spores. Due to its angioinvasive nature, it rapidly spreads from the nose to the palate, orbit, and eventually the brain, leading to tissue necrosis and characteristic black eschars. 2. **Why Incorrect Options are Wrong:** * **Option A:** Mucormycosis is highly prevalent in **India**, which has the highest incidence globally, especially following the COVID-19 pandemic (CAM - COVID-associated mucormycosis). * **Option C:** **Uncontrolled Diabetes Mellitus** (especially with Ketoacidosis) is the most significant predisposing factor. The acidic environment and high glucose levels promote fungal growth and impair phagocytic function. * **Option D:** Metachromatic bodies are characteristic of *Corynebacterium diphtheriae* (Volutin granules). Mucormycosis is identified by **broad, ribbon-like, non-septate (aseptate) hyphae** that branch at **right angles (90°)**. **High-Yield Clinical Pearls for NEET-PG:** * **Angioinvasion:** The hallmark of Mucor is its tendency to invade blood vessels, causing thrombosis and infarction. * **Diagnosis:** KOH mount or biopsy showing wide-angled branching. It stains poorly with Gram stain but well with **GMS (Grocott-Gomori Methenamine Silver)** and PAS. * **Treatment:** Surgical debridement is crucial, along with intravenous **Liposomal Amphotericin B**. Isavuconazole and Posaconazole are alternative agents.

Question 340: Which of the following is not a mould?

A. Aspergillus fumigatus
B. Rhizopus
C. Cryptococcus neoformans (Correct Answer)
D. Adsidia

Explanation: ### Explanation The classification of fungi is a high-yield topic for NEET-PG. Fungi are generally categorized into four morphological groups: **Moulds** (filamentous), **Yeasts** (unicellular), **Yeast-like** (yeast with pseudohyphae), and **Dimorphic** (exist as both forms depending on temperature) [2]. **Why Cryptococcus neoformans is the correct answer:** * **Cryptococcus neoformans** is a **true yeast** [1]. It exists exclusively as a unicellular budding cell and does not form hyphae or mycelium (mould form) [3]. * A defining feature of *Cryptococcus* is its thick **polysaccharide capsule**, which is best visualized using **India Ink** preparation (showing a clear halo) [1]. **Why the other options are incorrect:** * **Aspergillus fumigatus (Option A):** This is a classic **mould** characterized by septate hyphae that branch at acute angles (40-45°) [2], [4]. It is a common cause of Otomycosis and Aspergillosis. * **Rhizopus (Option B) and Absidia (Option D):** Both belong to the order Mucorales. These are **moulds** characterized by broad, non-septate (coenocytic) hyphae that branch at right angles (90°). They are notorious for causing Rhino-cerebral Mucormycosis, especially in uncontrolled diabetics. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic for Dimorphic Fungi:** "*Body Heat Probably Changes Shape*" (**B**lastomyces, **H**istoplasma, **P**aracoccidioides, **C**occidioides, **S**porothrix) [2]. 2. **Cryptococcus:** The most common cause of fungal meningitis in HIV/AIDS patients [1]. It is **Urease positive**. 3. **Culture:** Most fungi are grown on **Sabouraud Dextrose Agar (SDA)**. 4. **Mucor vs. Rhizopus:** *Rhizopus* has rhizoids (root-like structures) located directly below the sporangiophore, whereas *Absidia* has rhizoids located between sporangiophores (internodal).

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Mycology — MCQs

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