A 16 year-old presents for evaluation of a rash. The patient notes that it started on their back, with multiple spots, and has spread to their upper chest and arms. It itches a little. The patient also reports sweating more than before because their new summer job involves being outdoors. On physical examination, you note hyperpigmented, oval macules scattered more prominently around the upper back, chest, neck, and upper arms, as well as under the arms. Based on this description, what is the most likely diagnosis?

Superficial fungal skin infections are common in adolescents and young adults, especially in warm, humid environments or with increased sweating. These infections often affect the trunk and proximal upper extremities and may change skin pigmentation. Tinea versicolor is caused by overgrowth of Malassezia species on the skin, leading to hypo- or hyperpigmented macules with fine scaling. Heat, humidity, and sweating are important predisposing factors that worsen the condition. Rationale: A. Pityriasis rosea typically begins with a single “herald patch” followed by a generalized eruption of smaller oval lesions arranged in a “Christmas tree” pattern on the trunk. While it may involve the back and chest, it is usually self-limiting, mildly pruritic, and not associated with hyperpigmented macules or axillary involvement. The pigmentation changes and distribution described are more consistent with a fungal etiology. B. Tinea versicolor is the most likely diagnosis because it presents as multiple oval hypo- or hyperpigmented macules with fine scaling, commonly affecting the upper back, chest, neck, and upper arms. It is caused by overgrowth of Malassezia species, which thrive in warm, moist environments such as those created by increased sweating. Mild pruritus and distribution in seborrheic areas strongly support this diagnosis. C. Psoriasis is a chronic inflammatory skin condition characterized by well-demarcated erythematous plaques with silvery-white scale, commonly on extensor surfaces such as elbows and knees. It is not typically associated with hyperpigmented macules or prominent involvement of axillae in this pattern. The lesion description and distribution are not consistent with psoriasis. D. Atopic eczema presents with intensely pruritic, eczematous lesions that commonly affect flexural areas and are associated with dry skin and a history of atopy (asthma, allergic rhinitis). Lesions are usually erythematous, oozing, or lichenified rather than hyperpigmented oval macules. The morphology and distribution in this case do not align with atopic dermatitis.