A 36-year-old HIV-positive man (CD4 count 38 cells/uL) reports 6 weeks of painless, profuse, watery, non-bloody diarrhoea with up to 15 stools daily and significant weight loss. Stool examination by modified acid-fast staining reveals small round oocysts measuring 4 to 6 micrometres. Which organism is responsible?

• A Cyclospora cayetanensis
• B Giardia lamblia
• C Entamoeba histolytica
• D Cryptosporidium parvum ✓

Explanation

Cryptosporidium produces 4 to 6 micrometre acid-fast oocysts and causes prolonged secretory diarrhoea in advanced immunosuppression. No antimicrobial reliably eradicates it; the definitive response comes from restoring immunity with ART. This also kills the closest distractor: Cyclospora oocysts are larger, around 8 to 10 micrometres, and both Cyclospora and Cystoisospora infections respond to trimethoprim-sulfamethoxazole, unlike cryptosporidiosis.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.