A 27-year-old man who moved to Abu Dhabi from South Asia 6 months ago has widespread, itchy, ring-shaped plaques on the groin, buttocks and trunk. They did not clear after 6 weeks of oral terbinafine 250 mg daily, and several family members have similar lesions. A KOH mount of skin scrapings shows septate hyphae. Culture on Sabouraud agar with cycloheximide at 28 °C grows, in 7 days, a white, powdery colony with many round microconidia in grape-like clusters, and the laboratory identifies it by morphology as Trichophyton mentagrophytes complex. What should the laboratory do next?

Sequence the ITS region and test terbinafine susceptibility

Do a hair perforation test to exclude T. rubrum

Repeat the culture on dermatophyte test medium

Report T. mentagrophytes with no further testing

Molds and dermatophytes

10 DOH Abu Dhabi (HAAD) lab licensing practice questions on Molds and dermatophytes, part of Microbiology. The first 10 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 27-year-old man who moved to Abu Dhabi from South Asia 6 months ago has widespread, itchy, ring-shaped plaques on the groin, buttocks and trunk. They did not clear after 6 weeks of oral terbinafine 250 mg daily, and several family members have similar lesions. A KOH mount of skin scrapings shows septate hyphae. Culture on Sabouraud agar with cycloheximide at 28 °C grows, in 7 days, a white, powdery colony with many round microconidia in grape-like clusters, and the laboratory identifies it by morphology as Trichophyton mentagrophytes complex. What should the laboratory do next?
1. ASequence the ITS region and test terbinafine susceptibility
2. BDo a hair perforation test to exclude T. rubrum
3. CRepeat the culture on dermatophyte test medium
4. DReport T. mentagrophytes with no further testing
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Answer: A · Sequence the ITS region and test terbinafine susceptibility
Extensive, contagious tinea that fails terbinafine in a person from South Asia suggests Trichophyton indotineae, a member of the T. mentagrophytes complex that has spread from the Indian subcontinent to many countries. Its colonies and microscopy cannot be told apart from other members of the complex, so identification needs sequencing of the ITS region of ribosomal DNA. Most isolates carry mutations in the squalene epoxidase (SQLE) gene that cause terbinafine resistance, so antifungal susceptibility testing or SQLE sequencing is advised, often at a reference laboratory. Reporting 'T. mentagrophytes' alone would leave the cause of treatment failure undetected. The result also matters for public health, because the infection spreads within households.
- B: The morphology already excludes T. rubrum; the problem is terbinafine failure.
- C: DTM only shows that a dermatophyte is present and adds nothing here.
- D: This report would miss T. indotineae, which needs sequencing and susceptibility testing.
Question 2 · Hard
A 7-year-old girl in Abu Dhabi has patchy hair loss with black dots, and the Wood's lamp is negative. KOH of plucked hairs shows arthroconidia inside the hair shafts. After 3 weeks, the culture shows slow-growing, waxy, heaped, deep-purple colonies with almost no conidia. What is the most likely agent?
1. ATrichophyton tonsurans
2. BMicrosporum canis
3. CTrichophyton violaceum
4. DTrichophyton rubrum
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Answer: C · Trichophyton violaceum
Black-dot tinea capitis with arthroconidia inside the hair and a negative Wood's lamp points to an endothrix Trichophyton. T. violaceum grows very slowly as smooth, waxy, heaped violet colonies and rarely forms conidia; it is a common cause of tinea capitis in the Middle East and Africa. T. tonsurans, the other common endothrix species, forms suede-like colonies with a yellow to mahogany reverse and many microconidia of varied shape. Household contacts should be examined, because both species spread from person to person.
- A: T. tonsurans is also endothrix, but forms suede-like colonies with many microconidia.
- B: M. canis causes fluorescent ectothrix infection and forms many spindle-shaped macroconidia.
- D: T. rubrum rarely infects scalp hair and forms white, fluffy colonies with a wine-red reverse.
Question 3 · Easy
Rhino-orbital-cerebral mucormycosis occurs most often in patients with:
1. AChronic granulomatous disease
2. BAdvanced HIV infection
3. CDiabetic ketoacidosis
4. DCystic fibrosis
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Answer: C · Diabetic ketoacidosis
Mucorales such as Rhizopus thrive in acidic, high-glucose conditions with more free iron, which diabetic ketoacidosis provides, and acidosis also weakens neutrophil function. The fungus invades blood vessels and causes black, necrotic tissue in the nose, palate and orbit. Diabetes is common in the Gulf region, so laboratories should treat these specimens as urgent. Broad, ribbon-like hyphae with few septa on direct examination support the diagnosis.
- A: Chronic granulomatous disease predisposes mainly to Aspergillus infection.
- B: Advanced HIV predisposes mainly to cryptococcal and Pneumocystis infection.
- D: Cystic fibrosis is linked to Aspergillus colonization and allergic bronchopulmonary aspergillosis.
Question 4 · Easy
Which dermatophyte genus infects skin and nails but not hair?
1. AEpidermophyton
2. BMicrosporum
3. CNannizzia
4. DTrichophyton
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Answer: A · Epidermophyton
Epidermophyton floccosum, the only pathogenic species of its genus, infects skin and nails but never hair. It forms smooth, club-shaped macroconidia in clusters and no microconidia. Trichophyton species infect skin, hair and nails, while Microsporum species infect mainly skin and hair. The infected site therefore helps point to the likely genus.
- B: Microsporum species infect skin and hair, and rarely nails.
- C: Nannizzia species, such as N. gypsea (formerly Microsporum gypseum), infect skin and hair.
- D: Trichophyton species infect skin, hair and nails.
Question 5 · Medium
A mold from a sinus specimen grows quickly as yellow-green, granular colonies. Its conidiophores are rough-walled, and the round vesicles carry phialides over the whole surface. Which species fits best?
1. AAspergillus terreus
2. BAspergillus fumigatus
3. CPenicillium species
4. DAspergillus flavus
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Answer: D · Aspergillus flavus
Aspergillus flavus gives yellow-green colonies, rough conidiophores and round vesicles with phialides all over, in one or two rows. It is a common cause of fungal sinusitis in hot, dry climates, including the Middle East, and it also makes aflatoxins. A. fumigatus has smooth conidiophores with phialides only on the upper part of the vesicle. Identification to species matters because antifungal susceptibility differs between species.
- A: A. terreus is cinnamon-brown, with phialides on the upper vesicle in compact columns.
- B: A. fumigatus is blue-green, with smooth conidiophores and phialides on the upper vesicle only.
- C: Penicillium has brush-like branched conidiophores with no vesicle.
Question 6 · Medium
Dermatophyte test medium (DTM) turns from yellow to red around a colony because:
1. ADermatophytes ferment glucose and release acid
2. BThe cycloheximide reacts with fungal pigment
3. CDermatophytes use protein and release alkali
4. DDermatophytes reduce a tetrazolium dye to red
Answer and explanation in LearnFree account
Question 7 · Easy
An ear swab from a patient with otitis externa grows a mold whose colonies are white at first and then densely covered with black conidial heads, with a pale yellow reverse. Which species is most likely?
1. AAspergillus flavus
2. BAspergillus niger
3. CAspergillus terreus
4. DAspergillus fumigatus
Answer and explanation in LearnFree account
Question 8 · Hard
Sinus tissue from a man in diabetic ketoacidosis shows broad, ribbon-like hyphae with few septa on calcofluor white. The tissue was minced, not ground, and planted only on Sabouraud agar with cycloheximide and chloramphenicol. Nothing grows after 5 days. The most likely reason is:
1. AMucorales need 4 weeks before growth is seen
2. BChloramphenicol in the medium inhibits Mucorales
3. CCycloheximide in the medium inhibits Mucorales
4. DMucorales grow only under anaerobic conditions
Answer and explanation in LearnFree account
Question 9 · Easy
Infected hairs that fluoresce bright yellow-green under a Wood's lamp suggest tinea capitis caused by:
1. AEpidermophyton floccosum
2. BTrichophyton rubrum
3. CMicrosporum canis
4. DTrichophyton tonsurans
Answer and explanation in LearnFree account
Question 10 · Medium
Tissue shows narrow septate hyphae with regular branching at acute (about 45°) angles. Which fungus fits best?
1. ACandida
2. BCryptococcus
3. CMucorales (such as Rhizopus)
4. DAspergillus
Answer and explanation in LearnFree account

Molds and dermatophytes

Rhino-orbital-cerebral mucormycosis occurs most often in patients with:

Which dermatophyte genus infects skin and nails but not hair?

A mold from a sinus specimen grows quickly as yellow-green, granular colonies. Its conidiophores are rough-walled, and the round vesicles carry phialides over the whole surface. Which species fits best?

Dermatophyte test medium (DTM) turns from yellow to red around a colony because:

An ear swab from a patient with otitis externa grows a mold whose colonies are white at first and then densely covered with black conidial heads, with a pale yellow reverse. Which species is most likely?

Sinus tissue from a man in diabetic ketoacidosis shows broad, ribbon-like hyphae with few septa on calcofluor white. The tissue was minced, not ground, and planted only on Sabouraud agar with cycloheximide and chloramphenicol. Nothing grows after 5 days. The most likely reason is:

Infected hairs that fluoresce bright yellow-green under a Wood's lamp suggest tinea capitis caused by:

Tissue shows narrow septate hyphae with regular branching at acute (about 45°) angles. Which fungus fits best?