Review Question - QID 108738

A 68-year-old male presents to his primary care provider complaining of being tired. He reports gradually worsening fatigue over the past six months as well as intermittent night sweats. His medical history is notable for hyperlipidemia and diabetes mellitus. Physical exam is remarkable for painless cervical lymphadenopathy. Laboratory analysis is notable for a leukocyte count of 50,000/mm^3. A peripheral blood smear is obtained and shown in Figure A. He is started on a medication known to selectively target B cells. Which of the following cell surface markers is the most likely target of this medication?