**Core Concept**
Rotavirus is a major cause of severe diarrheal disease in infants and young children worldwide. It is a double-stranded RNA virus belonging to the Reoviridae family and is highly contagious.

**Why the Correct Answer is Right**
Rotavirus infection leads to gastroenteritis, characterized by severe diarrhea, vomiting, and fever. The virus primarily targets the small intestine, where it infects and damages the epithelial cells lining the intestinal villi. This results in a significant loss of electrolytes and fluids, leading to dehydration and other complications. The rotavirus genome contains 11 segments of double-stranded RNA, which are translated into six proteins necessary for viral replication and assembly.

**Why Each Wrong Option is Incorrect**
**Option A:** While adenovirus can cause gastroenteritis, it is not the primary cause of diarrheal disease in infants.
**Option B:** Norwalk virus (Norovirus) is a common cause of viral gastroenteritis in adults and older children but is less commonly associated with severe diarrheal disease in infants.
**Option D:** Hepatadenovirus is a rare and non-pathogenic virus that does not cause diarrheal disease in humans.

**Clinical Pearl / High-Yield Fact**
Infants with rotavirus infection often present with symptoms such as fever, vomiting, and watery diarrhea, which can lead to severe dehydration and electrolyte imbalances. Early recognition and treatment of rotavirus gastroenteritis are crucial to prevent complications and promote recovery.

**✓ Correct Answer: A. Rotavirus**