An otherwise healthy 37-year-old man has a routine full blood count done prior to donating blood for the first time. The results are:
|Mean corpuscular volume (MCV)||82 μm3|
|Mean corpuscular hemoglobin concentration (MCHC)||42%|
|White blood cell count||8,700/mm3|
A peripheral blood smear is shown in the picture below. On physical examination, his spleen is just palpable. Direct antiglobulin test (DAT) is negative. Which of the following best represents the etiology of this patient's condition?