Cryptosporidium

A 55-year-old man comes to urgent care for evaluation of chronic diarrhea. He has been having 5-10 episodes of watery diarrhea daily for the past 3 weeks. The patient returned from vacation in Eastern Europe last month. Medical history is notable for HIV and the patient has not been using antiretroviral therapy. Vitals are within normal limits. Physical examination reveals dry mucous membranes and oral thrush. Rectal examination is unremarkable. Microscopic examination of a stool sample after acid-fast staining is shown below. Which of the following is the most likely cause of this patient’s diarrhea?
 
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