Erythema on the thighs and buttocks with an annular scaly edge is characteristic of which condition?

• A. Psoriasis
• B. Seborrheic dermatitis
• C. Tinea cruris
• D. Eczema

Answer: (C)

The main idea here is recognizing a ring-shaped skin infection that expands outward and leaves a clearer center. That pattern is classic for a dermatophyte infection in the groin area, known as tinea cruris (jock itch). The key clue is an erythematous patch with an annular, scaly border—the border is the active edge where the fungus is growing, while the center clears as it heals.

Other common rashes don’t fit this ring with central clearing as well. Psoriasis tends to present as thick, well‑defined plaques with silvery scales on extensor surfaces or the scalp, not typically confined to the groin in a ring shape. Seborrheic dermatitis shows greasy scales in sebaceous areas like the scalp or face, not a ringed lesion in the groin. Eczema (atopic dermatitis) usually involves itchy, red, oozing or lichenified plaques in flexural areas rather than a ring with a distinct scaly border.

If there’s uncertainty, a simple test can help: a potassium hydroxide prep can reveal fungal hyphae, supporting the diagnosis of tinea cruris. Treatment is typically a topical antifungal.