Giardia and Cryptosporidium

A 28-year-old woman from Delhi presents with a 10-day history of profuse, watery diarrhea (6–8 stools per day), abdominal cramping, and bloating. She reports no fever or blood in stool. She recently returned from a trekking expedition in Himachal Pradesh where she drank untreated water from a mountain stream. On examination, she is mildly dehydrated but hemodynamically stable. Stool microscopy shows motile trophozoites with a characteristic pear-shaped appearance and two nuclei. What is the most likely causative organism?

A 35-year-old man with newly diagnosed HIV/AIDS (CD4 count 80 cells/μL) presents with severe, profuse watery diarrhea (15–20 stools per day) and abdominal pain for 3 weeks. He denies blood in stool or fever. Stool microscopy using modified acid-fast staining reveals small (4–6 μm), round oocysts. Stool culture for bacteria is negative. What is the most likely causative organism, and what is the most appropriate initial therapy?

Which of the following is the characteristic morphological feature that distinguishes Giardia lamblia trophozoites from other intestinal protozoa?