A 35-year-old male shoemaker presents with fever, cough, and gingival bleeding for 10 days, along with one episode of syncope. Physical exam: anemic appearance, scattered petechiae and ecchymoses on skin, 3-4 peanut-sized cervical lymph nodes (firm, mobile, no tenderness), marked tenderness at lower sternum, fine rales at lung bases, liver palpable 1.5 cm below costal margin, spleen palpable 3 cm below costal margin (firm consistency). The most diagnostic auxiliary test is: