This patient presents with this itchy rash on his legs after using cocaine.  His kidney function, ANCA, complement and viral serologies are all normal.  What is the likely diagnosis?

This patient likely has a cutaneous (or leukocytoclastic) vasculitis, secondary to cocaine or levamisole (an anti-helminthic agent which is commonly cut with cocaine). The classic dermatological findings of small vessel vasculitis are palpable purpura on the lower extremities. It is important, as in this case, to rule out end organ damage of vasculitis (renal & pulmonary beds are particularly prone to damage) and other causes of purpura with thorough examination and investigations.  Treatment is usually supportive and avoiding the triggering agent.