Question

A 46-year-old woman with HIV has severe persistent diarrhoea. On histological investigation, which organism is most likely causing the diarrhoea?

• A. Cryptosporidium
• B. Staphylococcus aureus
• C. Salmonella
• D. Clostridium botulinum

Hint:

Chronic watery diarrhoea plus low CD4, look for the acid-fast protozoan hugging the enterocyte brush border.

Answer 1

The Correct Option is A

Host-context approach. In an immunocompromised AIDS patient the differential for chronic diarrhoea is dominated by opportunists: Cryptosporidium, Microsporidia, Isospora, Cyclospora, CMV and Mycobacterium avium complex. The stem flags a histological diagnosis, which favours an organism with a distinctive tissue footprint. Cryptosporidium fits best: its oocysts stain acid-fast on modified Ziehl-Neelsen, and on biopsy the small round parasites cling to the apical surface of the enterocytes, an intracellular-but-extracytoplasmic location that is a board-favourite clue. Now discard the non-opportunists and the wrong syndromes: Staphylococcus aureus produces acute toxin-driven vomiting (not a chronic AIDS picture), Salmonella causes acute enterocolitis or enteric fever rather than histology-diagnosed persistent diarrhoea, and Clostridium botulinum causes flaccid paralysis with no diarrhoea at all. The immune context plus the brush-border parasite seals it. Answer: Cryptosporidium.