A 14-year-old boy presented with several years history for erythematous, scaly plaques on his trunk and extremities. The lesions were almost asymptomatic. There were no mucosal lesions and the skin of soles and palms were intact. He had not any other significant medical problem. 

What is your diagnosis? 

Diagnosis: Pityriasis Rubra Pilaris (PRP)

PRP is a rare, chronic disease of unknown etiology. PRP as generally recognized comprises more than one entitiy. In its classic form, the eruption usually starts as erythematous, slightly scaly plaques most often on the head, neck and trunk. A characteristic finding is sharply islands of unaffected skin.

Although "normal skin islands" were not typical in this case, but histopathology was in favour of PRP; absence of neutrophilic Munro microabscesses, short and thick rete ridges and thickened supra papillary plates were among these histopathological findings. 

 Omid Zargari, MD