Review Question - QID 214259

A 51-year-old man presents to his primary care physician with a 1-month history of increased bleeding. Specifically, he says that he has been developing hard to control nosebleeds at least once a week and seems to bleed more easily from other minor trauma. Furthermore, he says that he has been having some left leg pain located behind the knee. He has never had a problem with bleeding before this and has not made any major changes in his lifestyle. Past medical history is significant for hypertension and diabetes for which he is taking amlodipine and metformin. Physical exam reveals superficial bruising over bony prominences as well as 1+ edema of the left leg up to the level of the knee. The leg is mildly erythematous with no obvious rashes, and he has pain with compression of the left calf muscle. Labs are obtained with the following results:

Hemoglobin: 14.6 g/dL
Leukocyte count: 7,100 /mm^3 with normal differential
Platelet count: 527,000 /mm^3

Which of the following is the most likely cause of this patient’s condition?