Unusual Rash

An 8-year-old girl was accompanied by her mother, who complained of a rash on both her legs that she noticed several days earlier while playing outside. She had no other symptoms and denied any itching or pain. Her medical history included no significant conditions. Physical examination revealed macular, nonpalpable, extensive petechiae, and occasional patches of brownish pigmentation limited to her lower limbs, but the rest of her physical examination was normal. Lab results, including a complete blood cell count, erythrocyte sedimentation rate (ESR), and prothrombin and partial prothrombin time, were all within the normal range. A recent complete metabolic panel and urinalysis also showed no abnormalities. Which of the following diagnoses is most likely,