Mycology Practice Questions

Q: Which of the following organisms does not have a non-human reservoir?

Salmonella typhi. ### Explanation The core concept tested here is the **reservoir of infection**. A reservoir is the natural habitat (human, animal, or environmental) where an infectious agent lives and multiplies. **1. Why Salmonella typhi is correct:** *Salmonella typhi* and *Salmonella paratyphi* are **obligate human pathogens**. They have no known animal or environmental reservoir. Humans are the only hosts, and transmission occurs via the feco-oral route through contaminated food or water. This unique characteristic makes typhoid fever a candidate for potential eradication, unlike zoonotic diseases. **2. Why the other options are incorrect:** * **Yersinia pestis:** This is the classic example of a **zoonotic** pathogen. Its primary reservoirs are wild rodents (like rats, squirrels, and prairie dogs), and it is transmitted to humans via flea bites. * **Escherichia coli:** While many strains are part of the human normal flora, *E. coli* (especially STEC/EHEC) has significant reservoirs in **cattle** and other ruminants. It can also survive in environmental water sources. * **Clostridium tetani:** This organism has an **environmental reservoir**. It exists as highly resilient spores in **soil** and the intestinal tracts of grazing animals (like horses and cattle), which further contaminate the soil through feces. **Clinical Pearls for NEET-PG:** * **Chronic Carriers:** In *S. typhi*, the **gallbladder** is the most common site of long-term colonization (e.g., the famous case of "Typhoid Mary"). * **Zoonosis:** Diseases with animal reservoirs (like Plague, Rabies, or Brucellosis) are significantly harder to eradicate than human-only diseases (like Smallpox or Polio). * **Vi Antigen:** This capsular polysaccharide is a key virulence factor for *S. typhi* and is used in subunit vaccines.

Q: All of the following are true about Candida EXCEPT:

It is a mould. **Explanation:** The correct answer is **C (It is a mould)**. *Candida* species are primarily classified as **yeasts**, not moulds. **1. Why Option C is the correct "Except":** *Candida* is a unicellular fungus (yeast) that reproduces by budding. Unlike moulds, which are multicellular filamentous fungi consisting of true hyphae (e.g., *Aspergillus* or *Rhizopus*), *Candida* exists predominantly in yeast form. While it can form "pseudohyphae," it does not fit the structural definition of a mould. **2. Analysis of Other Options:** * **Option A (Pseudohyphae are seen):** This is **true**. Pseudohyphae are formed when buds fail to detach and elongate, creating a "sausage-link" appearance with distinct constrictions at the septa. (Note: *C. glabrata* is the only medically important species that does *not* form pseudohyphae). * **Option B (Produces chlamydospores):** This is **true**. Under specific conditions (e.g., growth on Cornmeal agar at 25°C), *Candida albicans* and *C. dubliniensis* produce thick-walled, resting spores called chlamydospores. * **Option D (Non-thermally dimorphic):** This is **true**. Unlike "true" dimorphic fungi (like *Histoplasma*) which change form based on temperature (yeast at 37°C, mould at 25°C), *Candida* is **pleomorphic**. It can form yeast, pseudohyphae, and true hyphae (germ tubes) within the same temperature range, often triggered by pH or nutrient changes. **High-Yield Clinical Pearls for NEET-PG:** * **Germ Tube Test (Reynolds-Braude Phenomenon):** The definitive rapid diagnostic test for *C. albicans*. * **C. glabrata:** Increasing in incidence; notably lacks pseudohyphae and is often resistant to azoles. * **C. auris:** A multi-drug resistant emerging pathogen causing hospital outbreaks. * **Culture:** Grows on Sabouraud Dextrose Agar (SDA) as creamy white, smooth colonies with a "yeasty" odor.

Q: What is the major cause of tinea capitis?

T. schoenlenii. **Explanation:** **Tinea capitis** is a dermatophyte infection of the scalp hair and skin. The correct answer is **T. schoenleinii** because it is the primary causative agent of **Favus**, a severe and chronic form of tinea capitis characterized by the formation of yellow, cup-shaped crusts called **scutula** and permanent scarring alopecia. In many clinical classifications and historical epidemiological data used in exams, *T. schoenleinii* is highlighted as a major specific cause of the inflammatory favosa variety. **Analysis of Options:** * **T. schoenleinii (Correct):** The classic cause of Favus. It shows an **endothrix** pattern of invasion and is identified by "favic chandeliers" (antler-like hyphae) on microscopy. * **M. canis:** A common cause of zoophilic tinea capitis (ectothrix), often transmitted from cats and dogs, but not the "major" global representative for chronic scalp infections in this context. * **E. floccosum:** This fungus **never infects hair**. It only affects the skin and nails (causing tinea cruris and tinea pedis). Therefore, it cannot cause tinea capitis. * **T. rubrum:** While it is the most common cause of dermatophytosis worldwide (tinea corporis and pedis), it is a **rare** cause of tinea capitis. **NEET-PG High-Yield Pearls:** 1. **Most common cause of Tinea capitis (Worldwide):** *T. tonsurans* (Endothrix). 2. **Favus Triad:** Scutula, scarring alopecia, and "favic chandeliers" on KOH mount. 3. **Wood’s Lamp:** *T. schoenleinii* shows a characteristic **pale green** fluorescence. 4. **Rule of Thumb:** *Epidermophyton* affects Skin + Nails; *Microsporum* affects Skin + Hair; *Trichophyton* affects Skin + Hair + Nails.

Q: In mycetoma, which organism produces pink to red granules?

Actinomadura pelletri. **Explanation:** Mycetoma is a chronic, granulomatous infection of the subcutaneous tissue characterized by a triad of localized swelling, underlying bone destruction, and the discharge of grains (granules) through sinus tracts. The **color and consistency of these granules** are high-yield diagnostic clues for identifying the causative agent. **Why Actinomadura pelletieri is correct:** * **Actinomadura pelletieri** is an aerobic actinomycete (Actinomycetoma) known for producing characteristic **small, firm, pink to red granules**. This is a classic "spotter" for NEET-PG. **Analysis of Incorrect Options:** * **Madurella (e.g., M. mycetomatis):** This is the most common cause of Eumycetoma (fungal). It produces **black granules** (Madura foot). * **Aspergillus:** While it can cause opportunistic infections, it is an uncommon cause of mycetoma. When it does occur, granules are typically **white to yellowish**. * **Actinomadura madurae:** This actinomycete produces **large, cream-colored to white/yellowish granules**, not red. **High-Yield Clinical Pearls for NEET-PG:** 1. **Granule Color Coding:** * **Red:** *Actinomadura pelletieri*. * **Black:** *Madurella mycetomatis*, *Exophiala jeanselmei*. * **Yellow/White:** *Nocardia brasiliensis*, *Actinomadura madurae*, *Streptomyces somaliensis*. 2. **Actinomycetoma vs. Eumycetoma:** Actinomycetoma (bacterial) usually has a more rapid progression and responds to antibiotics (Welsh regimen), whereas Eumycetoma (fungal) is slower and requires surgical debridement plus antifungals. 3. **Diagnosis:** The most definitive diagnosis is made by histopathological examination of the granules using **Gomori Methenamine Silver (GMS)** or **PAS stains**.

Q: Which fungus is typically not isolated in standard culture media?

Pneumocystis jiroveci. **Explanation:** The correct answer is **D. Pneumocystis jiroveci**. **Why it is correct:** *Pneumocystis jiroveci* is an atypical fungus that lacks ergosterol in its cell membrane and cannot be grown on standard fungal culture media (like Sabouraud Dextrose Agar). It is an **obligate extracellular parasite** that requires specific cell culture systems or animal models for growth, which are not used in routine clinical diagnostics. Diagnosis relies primarily on microscopic visualization (using GMS or Periodic acid-Schiff stains) of cysts or trophozoites in bronchoalveolar lavage (BAL) fluid or induced sputum, or via PCR. **Why the other options are incorrect:** * **A. Cryptococcus:** *Cryptococcus neoformans* grows readily on SDA and specialized media like Bird Seed Agar (Niger seed agar), where it produces characteristic brown-pigmented colonies due to phenoloxidase activity. * **B. Dermatophytes:** Fungi such as *Trichophyton*, *Microsporum*, and *Epidermophyton* are routinely isolated on SDA or Dermatophyte Test Medium (DTM), which contains phenol red as a pH indicator. * **C. Candida:** *Candida albicans* and other species are the most common fungi isolated in clinical labs. They grow rapidly (24–48 hours) on SDA and Blood Agar, forming creamy white colonies. **High-Yield Clinical Pearls for NEET-PG:** * **Stains for Pneumocystis:** Gomori Methenamine Silver (GMS) is the gold standard (stains cysts black/brown). * **Treatment of Choice:** Trimethoprim-Sulfamethoxazole (TMP-SMX), not standard antifungals, because it lacks ergosterol. * **CD4 Count:** Infection is typically seen in HIV patients when the CD4 count falls below **200 cells/mm³**. * **Radiology:** Characterized by bilateral "ground-glass opacities" on chest X-ray.

Q: Which statement is NOT true about Cryptococcus neoformans?

Is urease negative. **Explanation:** *Cryptococcus neoformans* is an encapsulated opportunistic yeast that is a frequent cause of meningitis, especially in immunocompromised patients (e.g., HIV/AIDS). **Why Option D is the correct answer:** *Cryptococcus neoformans* is **Urease positive**. This is a key biochemical characteristic used for its identification in the laboratory. The enzyme urease hydrolyzes urea into ammonia and carbon dioxide, increasing the pH and turning Christensen’s urea agar pink. Therefore, the statement "Is urease negative" is incorrect. **Analysis of other options:** * **Option A (Grows at 37°C):** Unlike many saprophytic fungi, *C. neoformans* is thermotolerant and grows well at 37°C. This ability to grow at human body temperature is a vital virulence factor. * **Option B (Grows on Sabouraud's agar):** It grows readily on Sabouraud Dextrose Agar (SDA), typically forming cream-colored, mucoid colonies (due to the capsule) within 48–72 hours. * **Option C (Has a polysaccharide capsule):** This is the most defining feature of *Cryptococcus*. The thick glucuronoxylomannan capsule is antiphagocytic and is visualized using **India Ink** (negative staining), where it appears as a clear halo against a dark background. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** Found in soil enriched with **pigeon droppings**. * **Diagnosis:** **Latex Agglutination test** for capsular antigen is more sensitive than India Ink. * **Culture:** On **Niger Seed Agar** (Birdseed agar), it produces brown/black colonies due to phenoloxidase activity (melanin production). * **Treatment:** Induction with Amphotericin B + Flucytosine, followed by Fluconazole.

Q: Aflatoxin is produced by which fungus?

Aspergillus flavus. ### Explanation **Correct Option: A. Aspergillus flavus** Aflatoxins are potent mycotoxins produced primarily by **Aspergillus flavus** and *Aspergillus parasiticus*. These fungi commonly contaminate stored agricultural crops like groundnuts (peanuts), maize, and rice under warm, humid conditions. **Medical Concept:** Aflatoxin B1 is the most potent natural carcinogen known. It undergoes metabolic activation in the liver to form an epoxide, which binds to DNA and causes a mutation in the **p53 tumor suppressor gene** (specifically a G to T transversion at codon 249). This significantly increases the risk of **Hepatocellular Carcinoma (HCC)**, especially in patients with chronic Hepatitis B infection. **Why other options are incorrect:** * **B. Aspergillus fumigatus:** This is the most common cause of invasive Aspergillosis, Aspergilloma (fungus ball), and Allergic Bronchopulmonary Aspergillosis (ABPA). It does not produce aflatoxins. * **C. Aspergillus niger:** Known for causing Otomycosis (fungal ear infection) and producing citric acid commercially. It is characterized by black-colored conidia but is not a source of aflatoxin. * **D. Penicillium marneffei (Talaromyces marneffei):** A dimorphic fungus prevalent in Southeast Asia, typically causing systemic infections in HIV-positive patients. It does not produce aflatoxins. **High-Yield Clinical Pearls for NEET-PG:** * **Target Organ:** Liver (Hepatotoxicity and Carcinogenicity). * **Detection:** Aflatoxins exhibit **fluorescence** under UV light (B1 and B2 fluoresce blue; G1 and G2 fluoresce green). * **Acute Toxicity:** Can lead to acute hepatic failure, presenting with jaundice and portal hypertension. * **Synergy:** There is a synergistic effect between Aflatoxin exposure and **Hepatitis B Virus (HBV)** in the pathogenesis of liver cancer.

Q: A HIV positive patient presents with cough for 10 days. Sputum culture by SDA reveals broad budding yeast. What is the diagnosis?

Blastomycosis. **Explanation:** The diagnosis is **Blastomycosis**, caused by the dimorphic fungus *Blastomyces dermatitidis*. The key diagnostic feature mentioned in the question is the presence of **broad-based budding yeast** (the daughter cell is attached to the parent cell by a wide neck). **Why Blastomycosis is correct:** In tissue or culture at 37°C (SDA), *Blastomyces* appears as large, thick-walled, spherical yeast cells. The characteristic "broad base" attachment between the bud and the parent cell is pathognomonic. It typically presents as a pulmonary infection (cough, fever) and is an opportunistic infection in immunocompromised individuals, such as HIV-positive patients. **Why other options are incorrect:** * **Histoplasma capsulatum:** Characterized by **small, intracellular yeast** cells (found within macrophages) with narrow-based budding. It does not show broad-based buds. * **Coccidioidomycosis:** Characterized by **spherules containing endospores** in tissue samples, not budding yeast. * **Paracoccidioidomycosis:** Characterized by large yeast cells with multiple buds, often described as a **"Pilot’s wheel"** or "Mickey Mouse" appearance, rather than a single broad-based bud. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Blastomyces:** **B**lasto = **B**ig, **B**lue, **B**road-**B**ased **B**uds. * **Geographic distribution:** Often associated with the Ohio and Mississippi River valleys (similar to Histoplasma). * **Systemic involvement:** In disseminated cases, it frequently involves the **skin** (verrucous lesions) and **bones**. * **Staining:** Best visualized using Silver stains (GMS) or PAS stain.

Question 1

A 50-year-old man presents with multiple abscesses and discharging sinuses on his left foot. X-ray of the foot shows destructive changes in the underlying bone. Which of the following is the most likely causative organism?

Accepted Answer

Nocardia

Answer

Sporothrix schenkii

Answer

Histoplasma capsulatum

Answer

Cryptococcus neoformans

Question 2

Which of the following organisms does not have a non-human reservoir?

Accepted Answer

Salmonella typhi

Answer

Yersinia pestis

Answer

Escherichia coli

Answer

Clostridium tetani

Question 3

All of the following are true about Candida EXCEPT:

Accepted Answer

It is a mould

Answer

Pseudohyphae are seen

Answer

It produces chlamydospores

Answer

It is a non-thermally dimorphic fungus

Question 4

What is the major cause of tinea capitis?

Accepted Answer

T. schoenlenii

Answer

M. canis

Answer

E. floccosum

Answer

T. rubrum

Question 5

In mycetoma, which organism produces pink to red granules?

Accepted Answer

Actinomadura pelletri

Answer

Madurella

Answer

Aspergillus

Answer

Actinumadura madurae

Question 6

Which fungus is typically not isolated in standard culture media?

Accepted Answer

Pneumocystis jiroveci

Answer

Cryptococcus

Answer

Dermatophyte

Answer

Candida

Question 7

Which statement is NOT true about Cryptococcus neoformans?

Accepted Answer

Is urease negative

Answer

Grows at 37°C

Answer

Grows on Sabouraud's agar

Answer

Has a polysaccharide capsule

Question 8

Aflatoxin is produced by which fungus?

Accepted Answer

Aspergillus flavus

Answer

Aspergillus fumigatus

Answer

Aspergillus niger

Answer

Penicillium marneffei

Question 9

A HIV positive patient presents with cough for 10 days. Sputum culture by SDA reveals broad budding yeast. What is the diagnosis?

Accepted Answer

Blastomycosis

Answer

Histoplasma capsulatum

Answer

Coccidiodomycosis

Answer

Paracoccidiomycosis

Question 10

A 20-year-old man presents with swelling of the left lower limb and hydrocele. Which of the following is NOT a causative agent for these symptoms?

Accepted Answer

Onchocerca volvulus

Answer

Brugia malayi

Answer

Brugia timori

Answer

Wuchereria bancrofti

Mycology — MCQs

Mycology — MCQs

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