**Question:** An 18-month old developed acute gastroenteritis with fever and watery diarrhea that lasted 10 days. She had been vaccinated against rotavirus at 2 and 4 months of age as recommended. Eight days later, both of her brothers, ages 3 and 6, developed acute gastroenteritis. Which of the following tests would most likely reveal the etiologic agent of their gastroenteritis?

A. Rotavirus ELISA test
B. Rotavirus PCR test
C. Stool culture for Salmonella
D. Stool culture for Shigella

**Correct Answer:** B. Rotavirus PCR test

**Core Concept:** Rotavirus is a common cause of acute gastroenteritis in children, particularly in the winter and spring seasons. It is the leading cause of severe dehydration and hospitalization in children worldwide. Rotavirus vaccination was introduced to prevent rotavirus gastroenteritis, and the vaccine used is rotavirus ELISA (Enzyme-Linked Immunosorbent Assay) test or rotavirus PCR (Polymerase Chain Reaction) test.

**Why the Correct Answer is Right:** The correct answer is B. Rotavirus PCR test. PCR (Polymerase Chain Reaction) is a highly sensitive and specific test that detects the viral genetic material of rotavirus. This test is superior to ELISA (Enzyme-Linked Immunosorbent Assay) test because PCR can detect very low levels of viral genetic material, even in the presence of other viruses or bacteria. This increases the chances of detecting the specific rotavirus causing the gastroenteritis in the siblings.

**Why Each Wrong Option is Incorrect:**

A. Rotavirus ELISA test (Option A) is also used for detecting rotavirus; however, PCR test is more sensitive and specific due to its ability to detect lower levels of viral genetic material.

B. While the siblings might have had rotavirus gastroenteritis, the answer is not explicitly stated in the question. We are only told about the 18-month-old's vaccination history, not the siblings'. It is possible that the PCR test can identify the strain of rotavirus in the siblings, but this information is not available in the question.

C. Stool culture for Salmonella (Option C) is used to identify bacterial infections, not viral infections. Rotavirus is a virus, not a bacterium, and stool culture is not suitable for detecting viral infections.

D. Stool culture for Shigella (Option D) is used for identifying bacterial infections, particularly Shigella species causing diarrhoea. Rotavirus is a virus and thus, a stool culture is not suitable for detecting viral infections.

**Clinical Pearls:**

The PCR test (Option B) is a highly sensitive and specific diagnostic tool for detecting viral infections, including rotavirus. In this case, it can identify the exact strain of rotavirus causing the gastroenteritis in the siblings, which is crucial for appropriate treatment and infection control measures in a family setting.