Pigmented scale with erythema on the abdomen

How would you determine the diagnosis?

Thomas, a 21-year-old Asian male, presents with a four-month history of a rash on his abdomen.


The rash is erythematous, pigmented, scaly and blanches. It is non-pruritic, and is exacerbated by sun exposure and sweating.

It has progressed to the midline of the anterior abdomen, left torso, and subsequently to the groin, scrotum and lower limbs with flexural sparing.

Of note, Thomas frequently uses the gym and travelled to Thailand three months after initial onset of the rash. He reports swimming there