A 30-year-old male presents with chronic, well-demarcated, coin-shaped (nummular) plaques on his lower legs and arms. The lesions are intensely pruritic, erythematous, and covered with small vesicles and crusts. He denies any history of atopy or recent exposure to new irritants. What is the most appropriate diagnosis?

Explanation

The description of chronic, well-demarcated, coin-shaped (nummular) plaques on the lower legs and arms, which are intensely pruritic, erythematous, and vesicular/crusted, is pathognomonic for nummular eczema (also known as discoid eczema). This condition is characterized by its distinctive round or oval lesions. B. Psoriasis vulgaris typically presents with well-demarcated erythematous plaques covered with silvery scales, often on extensor surfaces, and is usually less intensely pruritic than eczema. The morphology is different. C. Tinea corporis (ringworm) is a fungal infection that presents as annular lesions with an active, scaly border and central clearing. While it can be pruritic, the 'coin-shaped' description with vesicles and crusts is more indicative of eczema, and a fungal culture would be negative. D. Lichen planus presents with '6 Ps': pruritic, purple, polygonal, planar papules and plaques, often with Wickham's striae. The morphology is distinct from the described lesions.