Which stomach secretion, though small in overall digestive contribution, is described as the one stomach function that is absolutely necessary to life?

1. The stomach performs many digestive functions, including protein breakdown by HCl and pepsin. 2. However, only one stomach function is singled out as necessary to life itself: the production of intrinsic factor. 3. Intrinsic factor is required for intestinal absorption of vitamin B12, which is needed for red blood cell production and normal neurological function without a stomach, patients can survive with B12 injections instead. 4. HCl contributes to digestion and defense against bacteria but is not the one function identified as strictly necessary to life. 5. Pepsin helps digest protein but is likewise not the single life-necessary function being described. 6. Rennin helps infants digest milk protein but is a minor, age-limited function rather than the one necessary throughout life. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 23 "The Digestive System", section 23.4, p. 1113_