Mycology — MCQs

Mycology Indian Medical PG Practice Questions and MCQs

Question 531: A 40-year-old gardener presents with several subcutaneous nodules on his right hand, where he had cut himself on rose thorns, and physical examination reveals several erythematous fluctuant lesions. Which organism is most likely responsible for his condition?

A. Aspergillus
B. Malassezia
C. Sporothrix (Correct Answer)
D. Histoplasma

Explanation: ***Sporothrix*** - The gardener's history of a cut from rose thorns and the development of **subcutaneous nodules** are classic presentations of **sporotrichosis** (rose gardener's disease), caused by *Sporothrix schenckii*. - *Sporothrix schenckii* is a **dimorphic fungus** found in soil and on plant matter, causing localized cutaneous or subcutaneous lesions that typically follow **lymphatic spread** (lymphocutaneous pattern). *Aspergillus* - *Aspergillus* species typically cause **invasive pulmonary infections** (aspergillosis) in immunocompromised individuals or allergic bronchopulmonary aspergillosis. - While it can cause cutaneous infections, these are rare and usually occur in severely immunocompromised patients, without the classic "rose thorn" association. *Malassezia* - *Malassezia* species are yeasts that are normal skin flora and are primarily associated with **pityriasis versicolor**, **seborrheic dermatitis**, and **folliculitis**. - They do not typically cause deep subcutaneous nodules or are associated with puncture wounds from plants. *Histoplasma* - *Histoplasma capsulatum* is a **dimorphic fungus** that primarily causes **pulmonary infections** through inhalation of spores from soil contaminated with bird or bat droppings. - While it can rarely cause cutaneous lesions (especially in disseminated disease in immunocompromised patients), it is not associated with traumatic inoculation from plant material or the lymphocutaneous pattern seen here.

Question 532: What is the most probable point of entry for Aspergillus?

A. Puncture wound
B. Blood
C. Lungs (Correct Answer)
D. Gastrointestinal tract

Explanation: ***Lungs*** - The inhalation of **Aspergillus spores** (conidia) is the most common route of entry, leading to various forms of aspergillosis, especially in immunocompromised individuals. - Spores are ubiquitous in the environment and are easily aerosolized, making airborne transmission to the respiratory tract highly probable. *Puncture wound* - While fungal infections can occur via puncture wounds, this is a less common route for **Aspergillus** compared to airborne inhalation. - Cutaneous aspergillosis from direct inoculation is rare and typically seen in cases of severe trauma or surgical contamination in immunocompromised patients. *Blood* - **Aspergillus** does not typically originate in the blood; fungemia is usually a result of dissemination from a primary site of infection, most commonly the lungs, rather than a primary entry point. - Direct intravenous introduction of spores is highly uncommon and usually associated with contaminated medical devices or drugs rather than a natural point of entry. *Gastrointestinal tract* - The **gastrointestinal tract** is not a primary route of entry for **Aspergillus** in immune-competent individuals due to protective mechanisms like gastric acid and gut microbiota. - While ingestion of spores can occur, systemic infection originating from the GI tract is rare and generally seen only in severely immunocompromised patients with mucosal barrier damage.

Question 533: P. carinii primarily causes infection in which of the following?

A. Mice
B. Humans
C. Rabbits
D. Rats (Correct Answer)

Explanation: ***Correct: Rats*** - *Pneumocystis carinii* was initially identified as a distinct species primarily infecting **rats**. - This species is known to cause **pneumocystosis** in its specific host, the rat. - After taxonomic reclassification based on host specificity, the name *Pneumocystis carinii* was **retained for the rat-specific species**. *Incorrect: Mice* - While other *Pneumocystis* species can infect mice (e.g., *Pneumocystis murina*), *Pneumocystis carinii* specifically refers to the species found in **rats**. - The host specificity of *Pneumocystis* species is well-established, meaning each species typically infects only one host. *Incorrect: Humans* - Humans are primarily infected by ***Pneumocystis jirovecii***, which was formerly known as *Pneumocystis carinii* f. sp. *hominis*, highlighting its **human host specificity**. - *Pneumocystis jirovecii* causes **Pneumocystis pneumonia (PCP)**, especially in immunocompromised individuals. *Incorrect: Rabbits* - Rabbits are known to be infected by their own specific *Pneumocystis* species, *Pneumocystis oryctolagi*. - Infections in rabbits are typically asymptomatic, but can cause pneumonia in **immunocompromised** animals.

Question 534: Which of the following organisms plays an important role in the pathogenesis of "malt worker's lung"?

A. Aspergillus clavatus (Correct Answer)
B. Pseudomonas
C. Micropolyspora faeni
D. Aspergillus fumigatus

Explanation: ***Aspergillus clavatus*** - **Aspergillus clavatus** is the specific causative agent of **malt worker's lung**, a form of hypersensitivity pneumonitis (extrinsic allergic alveolitis) - It thrives in the **moist, warm conditions of germinating barley**, where malt workers are exposed to its spores during the malting process - This occupational exposure leads to an immunologic reaction in the lungs *Aspergillus fumigatus* - While *Aspergillus fumigatus* is a common cause of various aspergillosis syndromes (e.g., allergic bronchopulmonary aspergillosis, invasive aspergillosis), it is **not** typically associated with malt worker's lung - It is more broadly distributed in organic matter but not specifically linked to the malt industry in this context *Pseudomonas* - *Pseudomonas* species are **gram-negative bacteria**, often associated with nosocomial infections, cystic fibrosis, or chronic lung infections - They are **not fungi** and do not play a role in this specific allergic lung disease or hypersensitivity pneumonitis *Micropolyspora faeni* - *Micropolyspora faeni* (now reclassified as *Saccharopolyspora rectivirgula*) is the causative agent of **farmer's lung**, another type of hypersensitivity pneumonitis - It is found in **moldy hay**, not in malted barley, and therefore does not cause malt worker's lung

Question 535: Which of the following is the MOST COMMON fungal cause of Mucormycosis?

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

Explanation: ***Rhizopus oryzae*** - This is the **most common genus and species** responsible for mucormycosis infections in humans. - It belongs to the order **Mucorales**, which are characterized by broad, non-septate hyphae with a tendency to invade blood vessels. *Candida albicans* - This fungus is a common cause of **candidiasis**, which can manifest as thrush, vaginitis, or invasive candidemia. - It does not cause mucormycosis; its cellular morphology and disease presentation are distinct. *Aspergillus fumigatus* - This is the primary causative agent of **aspergillosis**, which can range from allergic reactions to invasive forms like chronic pulmonary aspergillosis or invasive aspergillosis. - While it can cause severe fungal infections, it is morphologically distinct (septate hyphae) and does not cause mucormycosis. *Cryptococcus neoformans* - This encapsulated yeast is best known for causing **cryptococcosis**, particularly **meningoencephalitis** in immunocompromised individuals. - Its disease presentation and microscopic features (yeast with budding, prominent capsule) are entirely different from those of Mucorales.

Question 536: Microscopy of a corneal ulcer shows branching septate hyphae. The probable diagnosis is

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

Explanation: ***Aspergillus*** - **Branching septate hyphae** observed on microscopy are characteristic of *Aspergillus* species, which are common causes of **fungal keratitis**, especially after ocular trauma or in immunocompromised individuals. - *Aspergillus* keratitis often presents as a **corneal ulcer** with distinct borders and satellite lesions, reflecting the invasive nature of the fungus. *Candida* - *Candida* species are typically observed as **yeast cells** that may form **pseudohyphae**, rather than true septate hyphae with dichotomous branching. - While *Candida* can cause keratitis, its microscopic appearance is distinct from the branching septate hyphae described. *Histoplasma* - *Histoplasma capsulatum* is a **dimorphic fungus** that primarily causes pulmonary infections and disseminated disease, particularly in immunocompromised patients. - It is typically seen as **small intracellular yeast forms** in macrophages, not as hyphae in corneal ulcers. *Mucormycosis* - **Mucorales** (e.g., *Mucor*, *Rhizopus*) cause **mucormycosis**, characterized by **broad, non-septate, ribbon-like hyphae** with irregular branching. - This morphology is distinctly different from the described **branching septate hyphae**, ruling out mucormycosis.

Question 537: What is the approximate pH of Sabouraud dextrose agar?

A. 12
B. 10
C. 5.4 (Correct Answer)
D. 8

Explanation: ***5.4*** - Sabouraud dextrose agar (SDA) is specifically formulated with an **acidic pH** range of approximately 5.0 to 6.0, most commonly cited as 5.4 to 5.6. - This **acidic environment inhibits the growth of most bacteria** while promoting the growth of fungi (yeasts and molds), making it a selective medium for fungal isolation. *12* - A pH of 12 is **highly alkaline**, which would be detrimental to the growth of most microorganisms, including fungi, and is not used for routine microbial culture. - Such a high pH would cause **denaturation of proteins** and essential cellular components, inhibiting cell function. *10* - A pH of 10 is also in the **alkaline range**, which is generally unfavorable for fungal growth as fungi prefer slightly acidic to neutral conditions. - Most fungi optimally grow at pH values **below 7**, and a pH of 10 would suppress their growth. *8* - A pH of 8 is slightly alkaline and would be **more favorable for bacterial growth** than fungal growth. - While some fungi can tolerate this pH, it is **not optimally selective** for fungi over bacteria, which is the primary purpose of SDA.

Question 538: Which organism is characterized by a wide polysaccharide capsule that appears as a clear halo around the yeast cell on India ink staining?

A. Penicillium marneffei
B. Blastomyces dermatitidis
C. Candida albicans
D. Cryptococcus neoformans (Correct Answer)

Explanation: ***Cryptococcus neoformans*** - This encapsulated yeast produces a **thick polysaccharide capsule** that is the key diagnostic feature - On **India ink preparation**, the capsule appears as a **clear halo** around the yeast cell because the ink particles cannot penetrate the capsule - This is the classic microscopic identification method for **Cryptococcus neoformans** in **cerebrospinal fluid** and other clinical specimens - The capsule can also be visualized using **mucicarmine stain** (stains red) or **alcian blue** *Penicillium marneffei* - This dimorphic fungus shows **intracellular yeast forms** with characteristic **central septation** (dividing by fission) - Typically found within macrophages and does not produce a prominent capsule - The yeast forms are oval to elongated with a transverse septum *Blastomyces dermatitidis* - Characterized by **large, thick-walled yeast cells** (8-15 μm) with **broad-based budding** - The cell wall is thick and refractile but there is **no polysaccharide capsule** - Budding occurs with a wide attachment point between mother and daughter cells *Candida albicans* - Forms **budding yeast cells**, **pseudohyphae**, and **true hyphae** (germ tubes) - Does not produce a polysaccharide capsule - Identified by germ tube formation at 37°C in serum

Question 539: A patient, a resident of Himachal Pradesh, presented with a series of ulcers in a row on his right leg. The biopsy from the affected area was taken and cultured on Sabouraud's dextrose agar. What is the most likely causative organism?

A. Cladosporium spp.
B. Pseudoallescheria boydii
C. Nocardia brasiliensis
D. Sporothrix schenckii (Correct Answer)

Explanation: ***Sporothrix schenckii*** - The presentation of "ulcers in a row" on the leg is highly suggestive of **lymphocutaneous sporotrichosis**, a characteristic finding where the infection spreads via lymphatic drainage. - This fungus is endemic in certain regions including parts of **Himachal Pradesh**, and is typically acquired through contact with contaminated soil or plant material (e.g., rose thorns, sphagnum moss). - Grows well on **Sabouraud's dextrose agar**, producing characteristic colonies. *Cladosporium spp.* - While *Cladosporium* can cause **phaeohyphomycosis** or allergic fungal sinusitis, it does not typically present with the classic lymphocutaneous lesions described. - These fungi are common environmental contaminants and their infections are usually associated with chronic skin lesions, not a linear spread of ulcers. *Pseudoallescheria boydii* - *Pseudoallescheria boydii* is a common cause of **mycetoma** (Madura foot), characterized by chronic, destructive lesions with granulomas and sinus tracts that discharge grains. - This presentation is distinct from the linear ulcerative lesions described in the patient. *Nocardia brasiliensis* - *Nocardia brasiliensis* is a bacterium (an actinomycete) that causes **actinomycetoma**, characterized by chronic, suppurative lesions with sinus tracts discharging grains. - The characteristic **"ulcers in a row"** (lymphocutaneous spread pattern) is **not typical** of Nocardia infection, which presents as localized mycetoma rather than ascending lymphatic involvement. - While Nocardia can grow on some fungal media, the clinical presentation is the key distinguishing feature here.

Question 540: Pityriasis versicolor is caused by which organism?

A. E. floccosum
B. M. gypseum
C. M. furfur (Correct Answer)
D. T. tonsurans

Explanation: ***M. furfur*** - **Pityriasis versicolor** is caused by **Malassezia furfur** (also known as Pityrosporum ovale), a dimorphic yeast that is a normal inhabitant of human skin. - Under certain conditions (e.g., humidity, excessive sweating), it overgrows, leading to characteristic hypopigmented or hyperpigmented patches. *E. floccosum* - **Epidermophyton floccosum** is a dermatophyte known to cause **tinea pedis (athlete's foot)**, tinea cruris (jock itch), and tinea unguium (nail infections). - It does not cause pityriasis versicolor, which is a superficial yeast infection, not a dermatophyte infection. *M. gypseum* - **Microsporum gypseum** is a geophilic dermatophyte, meaning it is found in soil and can cause **tinea corporis** or **tinea capitis** in humans, often through contact with contaminated soil. - It is not associated with pityriasis versicolor. *T. tonsurans* - **Trichophyton tonsurans** is an anthropophilic dermatophyte, a common cause of **tinea capitis** (ringworm of the scalp) and sometimes tinea corporis, particularly in children. - It causes specific types of dermatophytosis and is not the causative agent of pityriasis versicolor.

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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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