USMLE-Rx Step 2 Qmax Challenge #21049

Check out today’s Step 2 CK Qmax Question Challenge.

Know the answer? Post it in the comments below! Don’t forget to check back for an update with the correct answer and explanation (we’ll post it in the comments section below).

USMLE-Rx Step 2 Qmax Challenge #21049An adolescent boy is evaluated for a recent skin infection (as shown in the image). He is otherwise healthy and has never had a similar episode. He has a noncontributory social and family history. During the interview, he mentions he really enjoys working out at the community gym each day. On physical examination, he has no symptoms other than a localized infection that is pruritic and mildly malodorous.

Which of the following tests is best to confirm diagnosis?

A. Full-thickness biopsy
B. Potassium hydroxide preparation
C. Skin allergen test
D. Tzanck smear
E. Wood’s lamp


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8 thoughts on “USMLE-Rx Step 2 Qmax Challenge #21049”

  1. The correct answer is B. Tinea pedis (athlete’s foot) is a common cutaneous fungal infection that is often acquired secondary to occlusive footwear. It is often transmitted in swimming pool and shower facility areas. Although rare in younger children, it is common in preadolescents and adolescents. The infection often affects the lateral toe webs and the subdigital crevices. Interdigital fissures are often present. Chronic scaling (“moccasin ringworm”) involving extension onto the sole, sides of the foot, and in some cases the top of the foot follows. The border between involved and uninvolved skin is usually quite evident. A potassium hydroxide preparation will reveal branching hyphae. Treatment involves the application of topical antifungals.

    A is not correct. A biopsy is not needed in this case. It is useful in the work-up of cutaneous neoplasms, pigmented lesions, inflammatory lesions, and chronic skin disorders.

    C is not correct. Although contact dermatitis often presents as a localized pruritic rash, a skin allergen test is not necessary and would not help in diagnosis.

    D is not correct. Tzanck smear is a test for herpes.

    E is not correct. Wood’s lamp is used to diagnose tinea capitis, erythrasma, and porphyria cutanea tarda. Ultraviolet light is useful in the diagnosis of two species- Microsporum canis and audouini. Although both of these species fluoresce blue-green under a Wood’s lamp, they are not common causes of tinea infections. A Wood’s lamp is useful to differentiate between erythrasma caused by Corynebacterium minutissimum which fluoresces coral-red, from tinea cruris, which does not fluoresce. Erythrasma is a chronic superficial infection of the skin. It typically presents as macerated, scaly plaques between the toes or erythematous to dark colored patches or thin plaques in intertriginous areas. It is commonly asymptomatic but can at times be pruritic.

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