Clinical photograph of the elbow and skin biopsy from a patient with APL treated with ATRA and chemotherapy who developed a rash on day 18 of initial therapy. (A) Coalescing cluster of erythematous papules and pustules with scattered satellite papules on the upper extremity. (B) Histopathology at low power reveals a subepidermal blister with marked dermal edema and a mixed interstitial infiltrate composed of neutrophils and mononuclear cells with scattered nuclear debris. At high power, these cells appear to be immature myelomonocytic cells with some extravasated erythrocytes and nuclear debris.