A 28-year-old man is evaluated in the primary care office for new onset rash on both legs for 1 week. He is a healthy person with no past medical illness and is not taking any medications. There is no history of recent fever but he has noticed occasional gum bleeding and one episode of nosebleed 4 days ago. He does not smoke, drinks alcohol only socially, and does not use any recreational drug. On physical examination, he is a healthy appearing man with no lymphadenopathy or hepatosplenomegaly. Laboratory studies reveal hemoglobin 14.8 gm/dL, hematocrit 42, MCV 86 fL, leukocyte count 6,000/mL, and platelet 22,000/mL. Liver function and renal function studies are normal. Peripheral smear reveals decrease in platelet count and no red cell abnormalities. What is the most probable diagnosis?