Okay, let's tackle this question. The user provided a scenario involving a 3-month-old infant with symptoms of fever, vomiting, watery diarrhea, and signs of dehydration. The family members also have mild diarrhea. The key here is to figure out the cause of the infant's gastroenteritis and what diagnostic test is appropriate.

First, the core concept here is likely viral gastroenteritis, especially since multiple family members are affected. Rotavirus is a common cause in infants and often presents with these symptoms. The presence of mucus and white cells in the stool might be a bit confusing because bacterial infections like E. coli or Salmonella can cause similar findings. But in a viral case, you might still see some white cells due to inflammation, though typically not as many as in bacterial infections. However, the question mentions "sheets of white cells," which could suggest a bacterial etiology. Wait, but the options aren't given, so I need to think about the standard tests for such cases.

The correct diagnostic test for viral gastroenteritis is usually a stool test for the virus, like a rotavirus antigen test. If it's bacterial, you'd do a stool culture. But since the family has similar symptoms and the infant is young, rotavirus is more common. The presence of white cells could be due to the immune response even in a viral infection. Alternatively, maybe the question is testing the knowledge that in viral cases, the white cells are present but the test for the virus is the key.

Now, the options aren't provided, but common tests for gastroenteritis include stool culture, ova and parasite exam, C. difficile toxin, and viral antigen tests. Since the infant is 3 months old, parasites are less likely unless there's travel history. Bacterial causes like E. coli O157:H7 can cause dysentery with white cells, but the question mentions watery diarrhea, which is more typical of rotavirus. So the correct test would be a stool antigen test for rotavirus.

The incorrect options might include stool culture (which would be negative in viral cases), ova and parasites (unlikely here), or maybe a blood test (not typically needed for viral gastroenteritis). The hyperpnea could indicate metabolic acidosis from dehydration, but the main issue is the cause of the infection.

Clinical pearl: Rotavirus is a leading cause of severe diarrhea in infants and young children, and rapid antigen detection is the standard diagnostic test. Remember that viral gastroenteritis often presents with watery stools, while bacterial causes may have more blood or mucus, but exceptions exist. Also, the presence of multiple family members with symptoms points to a viral etiology.

**Core Concept**
This case involves acute gastroenteritis in an infant with systemic symptoms and signs of dehydration. The key differential diagnosis includes viral etiologies (e.g., rotavirus) versus bacterial infections (e.g., *Salmonella*, *Shigella*). Stool findings of mucus and white cells suggest inflammation, but viral infections can also present with such findings due to intestinal damage.

**Why the Correct Answer is Right**
The correct diagnostic test is **stool antigen detection for rotavirus**. Rotavirus is the most common cause of severe diarrhea in infants and young children, especially in outbreaks. It presents with fever, vomiting, and watery diarrhea, often without significant blood in stool. The presence of leuk

✓ Correct Answer: C. Blood and stool culture