A 40-year-old male presents with fatigue, petechiae, and easy bruising for the past two weeks. He denies fever, neurological symptoms, or recent infections. Physical examination reveals scattered petechiae but no hepatosplenomegaly or lymphadenopathy. His lab results are as follows: Platelet count: 70,000/µL, Hemoglobin: Low, WBC normal, Peripheral blood smear: no schistocytes. Which of the following is the most likely diagnosis?