Malaria parasites and life cycleDownloads

A 68-year-old man of Mediterranean descent comes to the clinic with complaints of fatigue for the past month. He reports that it is increasingly difficult for him to complete his after-dinner walks as he would get breathless and tired around 10 minutes. He endorses dizziness and an upper respiratory infection last week for which he “took a lot of aspirin.” Past medical history is significant for malaria 10 years ago (for which he was adequately treated with anti-malarial medications) and aortic stenosis status post prosthetic valve replacement 5 months ago. When asked if he has had similar episodes before, he claims, “Never! I’ve been as healthy as a horse until my heart surgery.” Physical examination is significant for mild scleral icterus bilaterally and a faint systolic murmur. Which of the following images represents a potential peripheral smear in this patient?

An investigator is studying the outcomes of a malaria outbreak in an endemic region of Africa. 500 men and 500 women with known malaria exposure are selected to participate in the study. Participants with G6PD deficiency are excluded from the study. The clinical records of the study subjects are reviewed and their peripheral blood smears are evaluated for the presence of Plasmodium trophozoites. Results show that 9% of the exposed population does not have clinical or laboratory evidence of malaria infection. Which of the following best explains the absence of infection seen in this subset of participants?