This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood Vessels, Subject name: Pathology
A 6-year-old girl presents with a 2-week history of a skin rash over her buttocks and legs and joint pain. The parents report seeing blood in the urine. Physical examination reveals palpable purpuric skin lesions and markedly swollen knees. The results of laboratory studies reveal abnormally high erythrocyte sedimentation rate (30 mm/h), BUN of 25 mg/dL, and serum creatinine of 3 mg/dL. Urinalysis demonstrates RBCs and RBC casts. The stool guaiac test is positive. Biopsy of lesional skin reveals deposits of IgA in the walls of small blood vessels. Which of the following is the most likely diagnosis? 
Select and explain. 
A. Henoch-Schonlein purpura, B. Hypersensitivity vasculitis, C. Kawasaki disease, D. Polyarteritis nodosa
A: Henoch-Schonlein purpura
Answer: A: Henoch-Schonlein purpura 
Explanation: Henoch-Schonlein purpura is the most common type of childhood vasculitis and is caused by vascular localization of immune complexes, containing predominantly IgA. Purpuric skin lesions and glomerulonephritis in a child suggest the diagnosis of Henoch-Schonlein purpura. The other choices do not include deposits of IgA in the blood vessels.Diagnosis: Henoch-Schonlein purpura