Mycology — MCQs

Mycology Indian Medical PG Practice Questions and MCQs

Question 501: Sabouraud's dextrose agar is primarily used for the isolation of which of the following?

A. Staphylococcus aureus
B. Aspergillus niger
C. Rhizopus stolonifer
D. Candida albicans (Correct Answer)

Explanation: ***Candida albicans*** - **Sabouraud's dextrose agar (SDA)** is the gold standard medium for **isolation of all fungi** (both yeasts and molds), formulated with low pH (5.6) and high glucose concentration to favor fungal growth while inhibiting most bacteria. - Among the fungal options listed, ***Candida albicans*** is the **best answer** because Candida species are the **most commonly encountered fungal pathogens** in clinical microbiology, making SDA most frequently used for their isolation in routine clinical practice. - While SDA supports growth of all fungi including Aspergillus and Rhizopus, **Candida species** (especially *C. albicans*) represent the majority of clinical fungal infections requiring laboratory diagnosis. *Staphylococcus aureus* - This is a **bacterium**, not a fungus. - The **low pH (5.6)** and high sugar content of SDA are specifically designed to **inhibit bacterial growth** while promoting fungal growth. - *Staphylococcus aureus* is isolated on bacterial media like **blood agar** or **mannitol salt agar (MSA)**, not fungal media. *Aspergillus niger* - This is a **filamentous fungus (mold)** that grows very well on SDA. - However, while *Aspergillus* species are important opportunistic pathogens, they are **less commonly isolated** in routine clinical microbiology compared to *Candida* species. - *Aspergillus* infections occur primarily in immunocompromised patients, whereas *Candida* infections are more frequent across all patient populations. *Rhizopus stolonifer* - This is a **zygomycete fungus (bread mold)** that can grow on SDA. - While *Rhizopus* species cause mucormycosis in susceptible hosts, these infections are **relatively uncommon** compared to candidiasis. - SDA supports *Rhizopus* growth, but this organism is not the primary target in routine clinical fungal isolation.

Question 502: Which fungus is primarily responsible for causing Ectothrix infections?

A. Trichophyton tonsurans
B. Microsporum canis (Correct Answer)
C. Trichophyton violaceum
D. Trichophyton mentagrophytes

Explanation: ***Microsporum canis*** - This fungus is a common cause of **tinea capitis** (scalp ringworm) and typically produces an **ectothrix** pattern of hair invasion. - In ectothrix infections, **arthroconidia** are formed on the **outside** of the hair shaft, damaging the hair cuticle. - *M. canis* is the **most common** cause of ectothrix infections, often transmitted from cats and dogs. *Trichophyton tonsurans* - This species is known for causing **endothrix** hair infections, where arthroconidia are formed **inside** the hair shaft, *without* affecting the cuticle. - It often leads to **black dot tinea capitis**, characterized by broken hair shafts at the scalp surface. *Trichophyton violaceum* - Similar to *Trichophyton tonsurans*, *Trichophyton violaceum* primarily causes **endothrix** hair infections. - It is associated with highly inflammatory scalp lesions and can also result in black dot tinea capitis. *Trichophyton mentagrophytes* - While this fungus **can** cause ectothrix infections, it is much **less common** than *Microsporum canis*. - *T. mentagrophytes* more frequently causes tinea corporis, tinea pedis, and tinea cruris rather than tinea capitis. - When it does affect hair, it may produce both ectothrix and endothrix patterns depending on the variant.

Question 503: Black piedra is caused by ?

A. Piedraia hortae (Correct Answer)
B. Trichosporon ovoides
C. Trichosporon inkin
D. Trichosporon asahii

Explanation: ***Piedraia hortae*** - **Black piedra** is a superficial fungal infection of the hair shaft, characterized by the formation of hard, dark brown to black nodules, and it is specifically caused by the ascomycetous fungus **_Piedraia hortae_**. - This fungus produces **ascospores** within the hair nodule, a key diagnostic feature. *Trichosporon asahii* - **_Trichosporon asahii_** is primarily associated with **white piedra** and systemic infections, particularly in immunocompromised individuals. - Unlike **black piedra**, white piedra manifests as soft, white to light brown nodules on the hair shaft. *Trichosporon ovoides* - **_Trichosporon ovoides_** is another species associated with **white piedra**, especially affecting scalp hair. - While it causes hair shaft infections, the characteristic nodules are soft and lighter in color, not the hard, dark nodules of black piedra. *Trichosporon inkin* - **_Trichosporon inkin_** is a species of **_Trichosporon_** known to cause **white piedra**, often affecting pubic hair. - It also causes systemic infections, but it is not the causative agent of **black piedra**.

Question 504: What is the primary component that dermatophytes infect?

A. Keratin (Correct Answer)
B. Epidermis
C. Hair
D. Nails

Explanation: ***Correct: Keratin*** - **Dermatophytes** are a specific type of fungi that have the unique ability to metabolize and survive on **keratin**, a fibrous structural protein found in skin, hair, and nails. - This **keratinolytic ability** allows them to infect and thrive in the outermost layer of the skin and its appendages. - Dermatophytes produce **keratinase enzymes** that break down keratin, making it their defining characteristic and primary nutritional source. *Incorrect: Epidermis* - While dermatophytes infect the **epidermis**, it is specifically the **keratinized cells** within the epidermis (stratum corneum) that they target, not the entire epidermal layer. - The epidermis itself is a complex structure with various cell types, but the primary nutritional source for dermatophytes within it is keratin. *Incorrect: Hair* - **Hair** is an appendage rich in keratin and can indeed be infected by dermatophytes, leading to conditions like **tinea capitis** (ringworm of the scalp). - However, keratin is the fundamental component in hair that dermatophytes utilize, making "keratin" a more encompassing and primary answer. *Incorrect: Nails* - **Nails** are also composed primarily of keratin and are a common site for dermatophyte infections, resulting in **onychomycosis**. - Similar to hair, while nails are a site of infection, the specific substance within them that dermatophytes target and rely upon is keratin.

Question 505: Which of the following statements about Pneumocystis jirovecii is correct?

A. Can be associated with CMV infection in some cases.
B. Usually diagnosed by examining respiratory secretions. (Correct Answer)
C. Can lead to Pneumatocele in some cases.
D. Primarily infects immunocompromised individuals.

Explanation: ***Usually diagnosed by examining respiratory secretions.*** - **Pneumocystis jirovecii** cannot be cultured on standard media, so diagnosis relies on microscopic examination of respiratory specimens. - **Bronchoalveolar lavage (BAL)** fluid, induced sputum, or transbronchial biopsy specimens are commonly used to identify the characteristic cysts or trophozoites using special stains like **Grocott's methenamine silver (GMS)** or **Giemsa**. *Can be associated with CMV infection in some cases.* - While both **Pneumocystis jirovecii pneumonia (PJP)** and **cytomegalovirus (CMV) pneumonia** are opportunistic infections that can affect immunocompromised individuals, they are distinct pathogens. - While co-infection can occur in individuals with severe immunosuppression, there isn't a direct causative association where PJP is specifically "associated with" CMV infection in a way that helps define PJP itself. *Can lead to Pneumatocele in some cases.* - **Pneumatoceles** (thin-walled, air-filled cysts in the lung parenchyma) are more commonly associated with **Staphylococcus aureus pneumonia** (especially in children) and sometimes with **P. jirovecii pneumonia** in severely immunocompromised individuals, but it's not a primary or defining feature of P. jirovecii infection. - Other pulmonary complications like **pneumothorax** are also observed in PJP, but pneumatocele formation is less frequent. *Primarily infects immunocompromised individuals.* - While **Pneumocystis jirovecii** *does* primarily cause disease in **immunocompromised individuals** (especially those with HIV/AIDS, organ transplant recipients, or those on immunosuppressive therapy), this statement alone is not the *most correct* or specific diagnostic criterion from the options provided. - Many infections primarily affect immunocompromised individuals, but the question asks for a correct statement about P. jirovecii, and its diagnosis through respiratory secretions is a key characteristic.

Question 506: Which of the following statements about Cryptococcus is false?

A. Urease negative (Correct Answer)
B. Urease positive
C. Primarily infects lung
D. Encapsulated yeast

Explanation: ***Urease negative*** - *Cryptococcus neoformans* is **urease positive**, not urease negative, meaning it produces the enzyme urease, which breaks down urea into ammonia and carbon dioxide. - The urease test is a **key biochemical test** used for laboratory identification of *Cryptococcus*. - This statement is **false** and therefore the correct answer to the question. *Urease positive* - *Cryptococcus neoformans* is indeed **urease positive**, a characteristic often used in its laboratory identification. - This is a **true statement** about *Cryptococcus*. *Encapsulated yeast* - *Cryptococcus neoformans* possesses a **thick polysaccharide capsule** that is its major virulence factor. - The capsule can be demonstrated using **India ink stain**, which shows a clear halo around the yeast cell. - This is a **true statement** about *Cryptococcus*. *Primarily infects lung* - *Cryptococcus neoformans* infection typically begins with **inhalation of spores** (basidiospores), leading to primary pulmonary infection. - While it can disseminate to the central nervous system (causing meningitis), the **lung is the primary site of infection**. - This is a **true statement** about *Cryptococcus*.

Question 507: Which of the following fungi is difficult to isolate in culture?

A. Candida
B. Cryptococcus
C. Dermatophytes
D. Malassezia furfur (Correct Answer)

Explanation: ***Malassezia furfur*** - *Malassezia furfur* is a **lipophilic fungus**, meaning it requires lipids (fatty acids) for growth, making it difficult to isolate on standard culture media. - Its requirement for **exogenous fatty acids** is a key reason for its fastidious nature in laboratory culture. *Candida* - **Candida species** are relatively easy to grow on standard mycological media like Sabouraud dextrose agar (SDA) and blood agar. - They are **opportunistic yeasts** that form creamy, smooth colonies, readily identifiable in routine clinical microbiology. *Cryptococcus* - **Cryptococcus neoformans** and *gatti* can be cultured using standard mycological media (e.g., SDA) and selective media like birdseed (Niger seed) agar, which helps in its identification due to melanin production. - They are known for their **capsule production**, a key virulence factor, and can be easily visualized in clinical specimens with India ink stain. *Dermatophytes* - **Dermatophytes** (e.g., *Trichophyton*, *Microsporum*, *Epidermophyton*) grow well on Sabouraud dextrose agar, often with the addition of antibiotics to inhibit bacterial growth. - Their ability to **colonize keratinized tissues** (skin, hair, nails) is characteristic, and they are not considered fastidious in culture.

Question 508: A farmer presents you with a cauliflower-shaped mass on foot, which developed after a minor injury. Microscopy shows copper penny bodies. What is the most likely diagnosis?

A. Sporotrichosis
B. Blastomycosis
C. Chromoblastomycosis (Correct Answer)
D. Phaeohyphomycosis

Explanation: **Chromoblastomycosis** - The characteristic "cauliflower-shaped" lesion on the foot following a minor injury, especially in a farmer (indicating outdoor exposure), is highly suggestive of chromoblastomycosis. - The presence of **copper penny bodies** (also known as **sclerotic** or **muriform cells**) on microscopy is **pathognomonic** for chromoblastomycosis. *Blastomycosis* - Blastomycosis typically presents with **granulomatous lesions** that can ulcerate but are rarely described as cauliflower-shaped. - Microscopic examination would reveal **broad-based budding yeast cells**, not copper penny bodies. *Sporotrichosis* - Sporotrichosis usually presents as **subcutaneous nodules** that can ulcerate and spread lymphatically, forming a chain of lesions. - Microscopy shows **cigar-shaped budding yeasts** within macrophages or neutrophils, which are distinct from copper penny bodies. *Phaeohyphomycosis* - Phaeohyphomycosis encompasses a broad group of infections by dematiaceous fungi that produce **dark-walled hyphae** or yeast-like cells in tissue. - While it can cause subcutaneous nodules or cysts, the presence of distinct copper penny bodies points away from phaeohyphomycosis as the primary diagnosis.

Question 509: Which special stain is used for identifying Cryptococcus?

A. ZN stain
B. Gram stain
C. Mucicarmine stain (Correct Answer)
D. Malachite green

Explanation: ***Mucicarmine stain*** - The mucicarmine stain is specific for identifying the **polysaccharide capsule** of *Cryptococcus neoformans* and *Cryptococcus gattii*. - The capsule stains a **bright red or pink**, making the organism easily visible against a pale background under light microscopy. *ZN stain* - **Ziehl-Neelsen (ZN) stain** is primarily used to identify **acid-fast bacilli**, particularly *Mycobacterium tuberculosis*. - It would not effectively visualize *Cryptococcus* species, as they lack the **mycolic acid** cell wall components that retain the stain. *Gram stain* - **Gram stain** is a differential stain used to classify bacteria based on their cell wall composition, distinguishing between **Gram-positive** and **Gram-negative** organisms. - While *Cryptococcus* is a yeast and can stain Gram-positive, this stain does not specifically highlight its unique capsular structure, making it less diagnostic. *Malachite green* - **Malachite green** is predominantly used as a counterstain in the **endospore staining technique** to visualize **bacterial endospores**. - It has no diagnostic utility for identifying *Cryptococcus* species.

Question 510: Fungal infection which is acquired by traumatic inoculation is?

A. Sporothrix (Correct Answer)
B. Coccidioides
C. Paracoccidioides
D. Blastomyces

Explanation: ***Sporothrix*** - **Sporotrichosis** is characteristically acquired through **traumatic inoculation** of the fungus, often from contact with soil, thorns, or decaying vegetation. - The organism causes **cutaneous lymphatic disease**, presenting as nodular lesions along lymphatic drainage paths. *Blastomyces* - **Blastomycosis** is typically acquired by inhaling airborne fungal spores, usually from **soil rich in organic matter** or decaying wood. - It primarily affects the **lungs** and can disseminate to the skin, bones, and other organs, but is not primarily associated with traumatic inoculation. *Coccidioides* - **Coccidioidomycosis** (Valley Fever) is acquired by inhaling **arthroconidia** present in dust or soil in endemic areas. - It is a **pulmonary infection** that can disseminate to other body sites, and its entry is almost exclusively respiratory, not traumatic. *Paracoccidioides* - **Paracoccidioidomycosis** is acquired by inhaling airborne fungal propagules, typically found in **soil in Latin America**. - It primarily causes **chronic pulmonary disease** and can spread to mucous membranes, skin, and lymph nodes, with no known association with traumatic inoculation.

Practice by Chapter

Classification of Fungi

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Superficial Mycoses

Practice Questions

Dermatophytes

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Subcutaneous Mycoses

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Candidiasis

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Aspergillosis

Practice Questions

Cryptococcosis

Practice Questions

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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