A 28-year-old homeless male with a past medical history significant for asthma comes to your clinic complaining of a chronic rash on his scalp and feet. He describes the rash as “dry and flaky,” and reports it has been present for at least a year. He was using a new dandruff shampoo he got over the counter, with little improvement. The patient reports it is extremely itchy at night, to the point that he can't sleep. On exam, you note a scaly patch of alopecia, enlarged lymph glands along the posterior aspect of his neck, and fine scaling in between his toes and on the heel and sides of his foot. His temperature is 99°F (37°C), blood pressure is 118/78 mmHg, and pulse is 81/min. Which of the following is the most accurate test for the suspected diagnosis?

KOH preparation of scalp scraping

Skin biopsy with histopathological examination

Culture on Sabouraud dextrose agar

A 17-year-old boy comes to the physician because of a nonpruritic rash on his chest for 1 week. He returned from a trip to Puerto Rico 10 days ago. He started using a new laundry detergent after returning. He has type 1 diabetes mellitus controlled with insulin. His mother has Hashimoto thyroiditis, and his sister has severe facial acne. Examination of the skin shows multiple, nontender, round, hypopigmented macules on the chest and trunk. There is fine scaling when the lesions are scraped with a spatula. Which of the following is the most likely underlying mechanism of this patient's symptoms?

Increased growth of Malassezia globosa

Infection with Trichophyton rubrum

Antigen uptake by Langerhans cells

Autoimmune destruction of melanocytes

Dermatophytes for USMLE Step 2 CK: High-Yield Microbiology Lessons

The Fungi - Keratin-Loving Critters

Dermatophytes are fungi causing superficial infections of skin, hair, and nails, collectively termed tinea (ringworm).
Pathogenesis relies on the enzyme keratinase, which digests the structural protein keratin for nutrients.
Three key anamorphic genera:
- - Trichophyton: Infects skin, hair, and nails.
- - Epidermophyton: Infects skin and nails.
- - Microsporum: Infects skin and hair.

KOH prep of skin scraping showing dermatophyte hyphae

⭐ Trichophyton rubrum is the most common dermatophyte isolate in humans, frequently causing athlete's foot (tinea pedis), jock itch (tinea cruris), and nail fungus (onychomycosis).

Clinical Syndromes - The Ring of Fire

Dermatophyte infections, or "tinea," are named by location. They cause pruritic, annular lesions with a raised, erythematous border and central clearing.

Tinea corporis lesions on arm

Location	Clinical Name	Key Features
Scalp/Hair	Tinea Capitis	Common in children. Presents with scaly patches, black dots (broken hairs), and potential alopecia. Severe form is a kerion: an inflamed, boggy, purulent lesion.
Body	Tinea Corporis	Classic "ringworm." Pruritic, annular lesion with a raised, scaly, advancing border and central clearing. Acquired from contact with humans, animals, or fomites.
Groin	Tinea Cruris	"Jock itch." Pruritic, erythematous patches with a well-demarcated border on the groin and inner thighs. Typically spares the scrotum.
Feet	Tinea Pedis	"Athlete's foot." Presents as interdigital maceration (most common), moccasin-type (dry, scaly soles), or vesiculobullous.
Nails	Tinea Unguium	Onychomycosis. Causes thickened, discolored (yellow/brown), and brittle nails with subungual debris.

Diagnosis & Rx - Kicking the Keratin-Eaters

Initial Diagnosis
- KOH Prep: Best initial test. Scrape lesion periphery, add 10-20% KOH. Look for branching septate hyphae.
- Wood's Lamp: UV-A light causes fluorescence in some species.
  - Microsporum species (e.g., M. canis) show a characteristic blue-green fluorescence.

KOH prep of dermatophyte showing septate hyphae

Confirmatory Culture
- Sabouraud Dextrose Agar: Gold standard for speciation. Slow growth (weeks).

⭐ Exam Favorite: Trichophyton species, the most common cause of tinea infections in humans, do not fluoresce under a Wood's lamp.

Treatment Strategy

High‑Yield Points - ⚡ Biggest Takeaways

Dermatophytes (Trichophyton, Epidermophyton, Microsporum) are fungi that metabolize keratin.

They cause superficial infections of the skin, hair, and nails, collectively known as tinea.

Diagnosis is made via KOH preparation of skin scrapings, which reveals septate hyphae.

Some Microsporum species can be identified by their fluorescence under a Wood's lamp.

Treatment is typically with topical antifungals, but oral therapy may be required for widespread infections or onychomycosis.

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Dermatophytes