A 5 year old male without medical history presents to the Emergency Department with his concerned mother with chief complaint of rash for 2 days. The patient has a palpable purpuric rash localized to the buttocks, legs, and arms (see below). The patient is well appearing and without history of other symptoms. The mom reports the patient had symptoms of a URI one week prior to arrival. The mom denies the patient having abdominal pain, vomiting, diarrhea, or blood in his urine. Other than symptomatic treatment and reassurance, what diagnostic test is appropriate in the Emergency Department?