A 9-month-old child is brought to the Health Depament to receive the second dose of oral polio vaccine, 2 weeks after the first vaccination. The child has mild diarrhea, so the decision is made to defer fuher immunizations. Bacteriologic examination of a stool culture is unremarkable; however, a small, single-stranded, positive RNA virus is isolated from the specimen. The viral isolate was not inactivated by ether. Which of the following viruses was most likely isolated?

**Core Concept**
The question tests the ability to identify a virus based on its characteristics, specifically its resistance to ether inactivation and its single-stranded RNA nature. This requires knowledge of viral properties and how they are used to classify and differentiate viruses.

**Why the Correct Answer is Right**
The virus isolated from the stool culture is a positive RNA virus that is resistant to ether inactivation. This characteristic is typical of the Norwalk virus, a small, single-stranded RNA virus that causes viral gastroenteritis. The Norwalk virus is a member of the Caliciviridae family and is known to be resistant to ether, which is used to inactivate lipid-containing viruses.

**Why Each Wrong Option is Incorrect**
* **Option A:** Rotavirus is a double-stranded RNA virus that is sensitive to ether inactivation, making it an unlikely candidate.
* **Option B:** Adenovirus is a double-stranded DNA virus that is not related to the virus described in the scenario.
* **Option C:** Norovirus is a close relative of the Norwalk virus and shares similar characteristics, but the question specifically mentions a Calicivirus, which is a different genus.
* **Option D:** Astrovirus is a single-stranded RNA virus, but it is not as commonly associated with viral gastroenteritis as the Norwalk virus.

**Clinical Pearl / High-Yield Fact**
The Norwalk virus is a leading cause of viral gastroenteritis in children and is often associated with outbreaks in daycare centers and other settings where close contact occurs. The virus is highly contagious and can be spread through the fecal-oral route.

**Correct Answer:** C. Norwalk virus

✓ Correct Answer: D. Poliovirus