Laboratory Diagnosis of Fungal Infections — MCQs
A 10-year-old boy presented with painful boggy swelling of scalp, multiple sinuses with purulent discharge, easily pluckable hair, and lymph nodes enlarged in occipital region, which of the following would be most helpful for diagnostic evaluation?
A child comes with a circular 3cm x 3cm scaly patchy hair loss with itching in the lesions. The investigation of choice is
Which test is most specific for diagnosing invasive aspergillosis?
Which of the following exists as yeast at 37°C and hyphae at 25°C?
The germ tube test is done for:
A biopsy of a lung nodule from a patient with a history of bird exposure reveals yeast cells with thick capsules. What is the most likely pathogen?
In a patient presenting with respiratory symptoms, acute angled septate hyphae are seen in which of the following fungi?
Beta 1,3 Glucan test is positive in all except?
Which of the following is the most common yeast pathogen?
A patient from the Ohio River valley presents with granulomatous prostatitis showing broad-based budding organisms on microscopy. What is the most likely infecting organism?
Laboratory Diagnosis of Fungal Infections Indian Medical PG Practice Questions and MCQs
Question 1: A 10-year-old boy presented with painful boggy swelling of scalp, multiple sinuses with purulent discharge, easily pluckable hair, and lymph nodes enlarged in occipital region, which of the following would be most helpful for diagnostic evaluation?
- A. Bacterial culture
- B. Biopsy and giemsa staining
- C. Patch test, gram staining and Tzank smear
- D. KOH mount (Correct Answer)
Explanation: ***KOH mount*** - The symptoms of **painful boggy scalp swelling**, **purulent discharge**, **easily pluckable hair**, and **occipital lymphadenopathy** in a child are highly suggestive of **Tinea capitis**, specifically **Kerion**. - A **KOH mount** is the most direct and rapid method to identify fungal elements (hyphae and spores) in hair shafts and scales, confirming the diagnosis of a dermatophyte infection. *Bacterial culture* - While there is **purulent discharge**, the primary presentation with **boggy swelling** and **hair loss** is more indicative of a fungal etiology. - A bacterial culture would only be useful to rule out secondary bacterial infection, but not as the initial diagnostic step for the described primary fungal condition. *Biopsy and Giemsa staining* - A **biopsy** is an invasive procedure and generally not the first-line diagnostic test for uncomplicated **Tinea capitis/Kerion**, where a non-invasive KOH mount is sufficient. - **Giemsa staining** is primarily used for identifying certain bacteria, parasites, or cellular morphology, but it is not the standard or most efficient method for diagnosing fungal infections of the hair. *Patch test, Gram staining and Tzank smear* - A **patch test** is used to diagnose **allergic contact dermatitis** and is irrelevant to the presented symptoms. - **Gram staining** is for bacterial identification, and a **Tzank smear** is used for viral infections like herpes, neither of which are indicated by the clinical picture of a fungal scalp infection.
Question 2: A child comes with a circular 3cm x 3cm scaly patchy hair loss with itching in the lesions. The investigation of choice is
- A. Tzanck smear
- B. Gram stain
- C. KOH mount (Correct Answer)
- D. Split skin smear
Explanation: ***Correct: KOH mount (Potassium Hydroxide mount)*** - A **KOH mount** is the investigation of choice for suspected **dermatophyte infections** (tinea capitis), which commonly present as circular, scaly patches of hair loss with itching in children. - It involves dissolving keratinous material to visualize **fungal hyphae** and spores directly under a microscope. - This is a quick, cost-effective, and highly specific first-line diagnostic test. *Incorrect: Tzanck smear* - A **Tzanck smear** is primarily used to diagnose **viral infections** like herpes simplex or varicella-zoster by identifying multinucleated giant cells. - It is not effective for detecting fungal elements responsible for scaly hair loss. *Incorrect: Gram stain* - A **Gram stain** is a technique used to classify **bacteria** based on their cell wall properties. - It would not reveal fungal hyphae or spores relevant to the described condition. *Incorrect: Split skin smear* - A **split skin smear** (or slit-skin smear) is typically used in the diagnosis of **leprosy** to identify acid-fast bacilli. - This technique involves scraping the dermis and is not suitable for diagnosing superficial fungal infections.
Question 3: Which test is most specific for diagnosing invasive aspergillosis?
- A. Culture
- B. Beta-D-glucan assay
- C. Microscopy
- D. Galactomannan test (Correct Answer)
Explanation: ***Galactomannan test*** - The **galactomannan test** detects a polysaccharide component of the *Aspergillus* cell wall, making it specific for *Aspergillus* species. - A positive result, especially in high-risk patients, strongly indicates **invasive aspergillosis** due to its high specificity. *Culture* - While culture can identify *Aspergillus*, it lacks specificity as it can grow as a **contaminant** in respiratory samples. - Recovery of *Aspergillus* from routine cultures does not always confirm invasive disease, often requiring additional evidence. *Beta-D-glucan assay* - The **beta-D-glucan assay** detects a pan-fungal cell wall component, meaning it is not specific to *Aspergillus*. - A positive result can indicate a wide range of **invasive fungal infections**, including candidiasis and pneumocystosis, but does not differentiate them. *Microscopy* - Microscopic examination can reveal **fungal elements** consistent with *Aspergillus* (e.g., septate hyphae with acute angle branching). - However, morphology alone is not definitive and requires confirmatory tests for species identification and to distinguish from other filamentous fungi.
Question 4: Which of the following exists as yeast at 37°C and hyphae at 25°C?
- A. Cryptococcus neoformans
- B. Histoplasma capsulatum (Correct Answer)
- C. Sporothrix globosa
- D. Candida albicans
Explanation: ***Histoplasma capsulatum*** - This fungus is **thermally dimorphic**, growing as **yeast** at body temperature (37°C) in the host and as **hyphae (mold)** at cooler temperatures (25°C). - This is the **classic example** taught for thermal dimorphism in medical mycology. - Causes **histoplasmosis**, endemic in Ohio and Mississippi river valleys; presents with pulmonary disease and can disseminate in immunocompromised patients. - Laboratory identification relies on this characteristic temperature-dependent growth pattern. *Sporothrix globosa* - This fungus is also thermally dimorphic with the same pattern: **yeast at 37°C** and **hyphae at 25°C**. - Causes **sporotrichosis** (rose gardener's disease), typically presenting as lymphocutaneous infection. - While medically correct, **Histoplasma capsulatum** is the more commonly referenced example in standard medical textbooks for this specific thermal dimorphism pattern. *Cryptococcus neoformans* - Exists primarily as an **encapsulated yeast** at both temperatures. - Does **not** exhibit thermal dimorphism; remains in yeast form in both host and environment. *Candida albicans* - Exhibits dimorphism but grows as **yeast** at most temperatures and forms **pseudohyphae/germ tubes at 37°C** in response to serum and other factors. - Does not follow the classic thermal dimorphism pattern of yeast at 37°C and mold at 25°C.
Question 5: The germ tube test is done for:
- A. Candida albicans (Correct Answer)
- B. Neisseria gonorrhoeae
- C. Bacterial vaginosis
- D. Chlamydia
Explanation: ***Candida albicans*** - The **germ tube test** is a rapid screening method used to differentiate *Candida albicans* from other *Candida* species and yeasts. - *Candida albicans* produces **germ tubes** (true hyphae without constrictions at the base) when incubated in serum at 37°C for 2-3 hours. *Chlamydia* - **Chlamydia** species are **obligate intracellular bacteria** and are identified using molecular methods (e.g., NAAT), culture, or immunofluorescence, not the germ tube test. - They do not form structures like germ tubes, as they are prokaryotic organisms. *Bacterial vaginosis* - **Bacterial vaginosis** is a dysbiosis of the vaginal flora, diagnosed based on Amsel criteria (e.g., clue cells, pH >4.5, positive whiff test) or Nugent score from Gram stain. - It involves an overgrowth of various anaerobic bacteria and a decrease in lactobacilli, none of which produce germ tubes. *Neisseria gonorrhoeae* - **Neisseria gonorrhoeae** is a Gram-negative bacterium, typically identified by Gram stain showing intracellular **Gram-negative diplococci** within neutrophils, and subsequent culture or NAAT. - As a bacterium, it does not form germ tubes; this test is specific to certain fungi.
Question 6: A biopsy of a lung nodule from a patient with a history of bird exposure reveals yeast cells with thick capsules. What is the most likely pathogen?
- A. Aspergillus fumigatus
- B. Blastomyces dermatitidis
- C. Histoplasma capsulatum
- D. Cryptococcus neoformans (Correct Answer)
Explanation: ***Cryptococcus neoformans*** - The presence of **yeast cells with thick capsules** is a classic histological finding for *Cryptococcus neoformans*. - While *Cryptococcus* commonly affects immunocompromised individuals, it can also be found in **bird droppings**, particularly from pigeons, making the history of bird exposure relevant. *Aspergillus fumigatus* - *Aspergillus fumigatus* typically presents as **hyphae**, not yeast cells, and would not have a thick capsule. - Infections often manifest as **aspergillomas** (fungus balls) in lung cavities or invasive disease in immunocompromised patients. *Blastomyces dermatitidis* - *Blastomyces dermatitidis* appears as **large, broad-based budding yeast cells** but does not possess a thick capsule. - It is typically found in the **soil**, especially in moist areas, and its association with bird exposure is not as strong as with *Cryptococcus*. *Histoplasma capsulatum* - *Histoplasma capsulatum* is characterized by **small intracellular yeast forms** within macrophages and does not have a thick capsule. - It is strongly associated with **bird and bat droppings** but its microscopic appearance is distinct from that described.
Question 7: In a patient presenting with respiratory symptoms, acute angled septate hyphae are seen in which of the following fungi?
- A. Aspergillus (Correct Answer)
- B. Mucor
- C. Penicillium
- D. Candida
Explanation: ***Aspergillus*** - *Aspergillus* species are characterized by their distinctive **acute-angled (dichotomous) septate hyphae** when observed microscopically in tissue sections or clinical samples. - This morphological feature is crucial for differentiating *Aspergillus* infections, such as **aspergillosis**, from other fungal infections. *Mucor* - *Mucor* (along with *Rhizopus* and *Lichtheimia*) belongs to the Zygomycetes class, which are characterized by **broad, non-septate, ribbon-like hyphae** with irregular branching, not acute-angled septate hyphae. - These fungi are associated with **mucormycosis**, often seen in immunocompromised patients, particularly those with diabetes. *Penicillium* - *Penicillium* is a common mold known for producing penicillin and typically appears as **septate hyphae** but is more often recognized by its branching, brush-like conidiophores (penicilli) in culture rather than a distinct acute-angled septate hyphal morphology in clinical samples causing invasive disease. - While it can cause opportunistic infections, its hyphae are less commonly described as having uniquely acute angles compared to *Aspergillus*. *Candida* - *Candida* species are typically observed as **yeast cells (oval budding forms)**, often forming **pseudohyphae** (elongated yeast cells that remain attached) or true hyphae under specific conditions, but not as acute-angled septate hyphae. - *Candida* is a common cause of superficial and invasive candidiasis, and its microscopic appearance is distinct from filamentous fungi.
Question 8: Beta 1,3 Glucan test is positive in all except?
- A. Pneumocystis Jirovecii
- B. Candida
- C. Mucormycosis (Correct Answer)
- D. Aspergillus
Explanation: ***Mucormycosis*** - Fungi causing mucormycosis belong to the order **Mucorales**, which structurally lack **beta-D-glucan** in their cell walls. - Due to the absence of beta-D-glucan, the **beta-1,3-D-glucan assay** will yield a negative result in cases of mucormycosis. *Pneumocystis jirovecii* - This fungus contains significant amounts of **beta-D-glucan** in its cell wall, making the test usually positive during active infection. - A positive **beta-D-glucan test** can be a useful diagnostic marker for **Pneumocystis pneumonia (PCP)**, especially in immunocompromised patients. *Candida* - **Candida species** possess a cell wall rich in **beta-D-glucan**, leading to a positive test result during active infection. - The **beta-D-glucan assay** is a valuable adjunctive test for diagnosing invasive candidiasis. *Aspergillus* - The cell wall of **Aspergillus** contains **beta-D-glucan**, causing the test to be positive in cases of invasive aspergillosis. - A positive **beta-D-glucan test** can aid in the early diagnosis and management of invasive aspergillosis, particularly in high-risk patients.
Question 9: Which of the following is the most common yeast pathogen?
- A. Candida (Correct Answer)
- B. Mucor
- C. Rhizopus
- D. Cryptococcus
Explanation: ***Candida*** - **Candida** species, particularly *Candida albicans*, are the **most common cause of fungal infections** worldwide, ranging from superficial mucocutaneous infections to invasive systemic candidiasis. - They are part of the normal human microbiota and opportunistic pathogens, thriving in conditions like **immunocompromise**, antibiotic use, or presence of medical devices. *Mucor* - **Mucor** is a genus of mold, not yeast, and is known to cause **mucormycosis**, a severe and rapidly progressive fungal infection. - While dangerous, mucormycosis is **much rarer** compared to candidiasis. *Rhizopus* - **Rhizopus** is also a genus of mold, not yeast, and is another causative agent of **mucormycosis**, particularly in immunocompromised individuals. - Like Mucor, infections caused by Rhizopus are **less common** than those caused by Candida. *Cryptococcus* - **Cryptococcus neoformans** is a significant yeast pathogen, primarily causing **cryptococcosis**, which often manifests as meningoencephalitis in immunocompromised patients. - Although it is an important pathogen, its overall incidence is **lower than that of Candida** species.
Question 10: A patient from the Ohio River valley presents with granulomatous prostatitis showing broad-based budding organisms on microscopy. What is the most likely infecting organism?
- A. Escherichia coli
- B. Histoplasma capsulatum
- C. Pseudomonas aeruginosa
- D. Blastomyces dermatitidis (Correct Answer)
Explanation: ***Blastomyces dermatitidis*** - This organism is endemic to the **Ohio River valley** and is known to cause **granulomatous inflammation**, including in the prostate. - Its characteristic morphology is **broad-based budding yeast** on microscopy, fitting the description. *Escherichia coli* - While *E. coli* is a common cause of **bacterial prostatitis**, it does not present as granulomatous inflammation. - It is a **bacterium**, not a fungus, and would not show broad-based budding organisms. *Histoplasma capsulatum* - **Histoplasma** is also endemic to the Ohio River valley and causes granulomatous disease, but its yeast forms are **small**, intracellular, and do not exhibit broad-based budding. - It is often associated with a history of exposure to **bat or bird droppings** and commonly affects the lungs. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* causes **bacterial infections**, often in immunocompromised individuals or associated with catheter use, and is not a fungus. - It causes **acute inflammation**, not chronic granulomatous inflammation, and does not show broad-based budding.
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