**Question:** An infant was brought to the casualty with complaints of watery diarrhoea and vomiting. He had mild fever. What is the MOST common cause of acute infantile gastroenteritis?

A. Rotavirus
B. Norovirus
C. Adenovirus
D. Enteric fever (Salmonella typhi)

**Core Concept:** Acute infantile gastroenteritis is a condition characterized by sudden onset of diarrhoea, vomiting, and sometimes fever in infants. The most common causes can be broadly categorized into viral, bacterial, and parasitic infections.

**Why the Correct Answer is Right:** The correct answer, **Rotavirus**, belongs to the family Reoviridae and is the leading cause of acute gastroenteritis in infants globally. Rotavirus infection causes inflammation of the intestinal lining, leading to watery diarrhoea, vomiting, and fever. Although there are several strains, only two types (A and B) are responsible for most cases.

**Why Each Wrong Option is Incorrect:**

A. **Norovirus** is a viral pathogen but is not the most common cause of infantile gastroenteritis. It affects adults more commonly and presents with acute onset diarrhoea, vomiting, and sometimes fever.

B. **Adenovirus** is another viral pathogen, but it is less common than rotavirus in causing infantile gastroenteritis. It presents with symptoms similar to rotavirus infections.

D. **Enteric fever (Salmonella typhi)** is a bacterial infection caused by Salmonella typhi, which is responsible for typhoid fever. While typhoid fever can present with gastrointestinal symptoms, it is not the most common cause of acute infantile gastroenteritis.

**Clinical Pearl:** While understanding the different pathogens responsible for infantile gastroenteritis is essential for proper diagnosis and treatment, it is crucial to be aware of clinical presentations and differential diagnoses in such cases. A thorough history, physical examination, and appropriate investigations (stool tests) can help differentiate between the various causes of gastroenteritis in infants.