Palpable purpura and a visible sock line.

A 21-year-old woman came to the clinic, frightened by a painful purpuric rash on her lower extremities. The lesions appeared suddenly 3 days before, with no prior similar episodes. The pain, and some swelling that happened when she stood, had finally driven her to take some time off from her job and seek medical advice. She was diagnosed with a case of pharyngitis earlier that week; due to multiple drug allergies, she was prescribed a course of clindamycin. She had not experienced any nausea or vomiting, fever, abdominal cramping, or gross hematuria. On examination, the patient was friendly and good-humored, although she was concerned about her rash and visibly uncomfortable. She was walking with the aid of a borrowed cane, but her lesions were no longer tender to palpation. The rash consisted mainly of purpuric papules almost entirely limited to her legs, although some isolated lesions were on her back as well. The papules were concentrated around her distal lower extremities, with a clear line of lesions encircling her calves bilaterally where her knee-high socks had applied pressure for the last 2 days. Mild edema was noted, but the rest of her physical exam was normal. By dipstick, the patient had blood in her urine but no protein. What is the diagnosis? What is the treatment for this condition?