A 31-year-old HIV-positive man (CD4 count 65 cells/μL) has 6 weeks of profuse non-bloody watery diarrhoea with cramping abdominal pain, unresponsive to antibiotics. Modified acid-fast staining of stool reveals small round oocysts measuring 4 to 6 micrometers. What is the causative organism?
- A Entamoeba histolytica
- B Cryptosporidium parvum ✓
- C Giardia lamblia
- D Isospora belli
Correct answer: B. Cryptosporidium parvum
Explanation
Chronic watery diarrhoea in advanced AIDS with tiny acid-fast oocysts of 4 to 6 micrometers identifies Cryptosporidium parvum. The disease is self-limited in immunocompetent hosts but causes prolonged secretory diarrhoea below CD4 100, and effective antiretroviral therapy restoring immunity is the mainstay of cure. Isospora belli produces larger acid-fast oocysts around 20 to 30 micrometers, Giardia causes fatty malodorous stools with trophozoites, and Entamoeba produces bloody dysentery with amoebic ulcers.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.