A 28-year-old man develops profuse watery diarrhoea two days after eating food at a roadside stall during travel. Stool culture grows E. coli identified as enterotoxigenic E. coli (ETEC) producing only the heat-stable toxin (STa). The mechanism by which STa causes diarrhoea is:
- A Activation of guanylate cyclase increasing cyclic GMP ✓
- B ADP-ribosylation of Gs alpha increasing cyclic AMP
- C Inactivation of the 60S ribosomal subunit
- D Insertion into the host membrane forming pores
Correct answer: A. Activation of guanylate cyclase increasing cyclic GMP
Explanation
STa binds the guanylate cyclase C receptor on enterocytes, activating it directly and raising intracellular cyclic GMP. Cyclic GMP inhibits sodium chloride absorption and stimulates chloride secretion through CFTR channels, producing watery diarrhoea. Option B describes heat-labile toxin (LT), which is ADP-ribosylating and cyclic AMP dependent, and is the classic distractor here. Ribosomal inactivation belongs to Shiga toxin and pore formation to RTX-type haemolysins.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.