**Core Concept**
Enterotoxigenic E. coli (ETEC) is a pathogenic strain of E. coli that causes traveler's diarrhea, characterized by the production of heat-labile (LT) and/or heat-stable (ST) enterotoxins. These toxins act on the small intestine, leading to increased fluid secretion and electrolyte loss.

**Why the Correct Answer is Right**
ETEC is a major cause of diarrhea in travelers to developing countries, particularly in areas with poor sanitation and hygiene. The heat-labile toxin (LT) of ETEC works by activating the adenylate cyclase enzyme in the intestinal mucosa, resulting in increased levels of cyclic AMP (cAMP) and subsequent secretion of fluid and electrolytes into the intestinal lumen. The heat-stable toxin (ST) of ETEC, on the other hand, activates guanylate cyclase, leading to increased levels of cyclic GMP (cGMP) and similar effects.

**Why Each Wrong Option is Incorrect**
**Option A:** ETEC is not a major cause of diarrhea in immunocompromised patients. While ETEC can cause severe diarrhea in anyone, it is not specifically associated with immunocompromised individuals.
**Option B:** ETEC does not typically cause bloody diarrhea. While ETEC can cause severe diarrhea, it usually does not result in bloody stools.
**Option C:** ETEC is not a major cause of diarrhea in children under 5 years old. While ETEC can cause diarrhea in anyone, it is not a major cause of diarrhea in young children.

**Clinical Pearl / High-Yield Fact**
ETEC is a significant cause of traveler's diarrhea, and its prevention is crucial for individuals traveling to developing countries. The use of oral rehydration therapy (ORT) and antibiotics, such as rifaximin, can help manage ETEC infections.

**Correct Answer: C.**