**Core Concept**
The diagnosis of rotavirus diarrhea primarily relies on clinical presentation, epidemiological clues, and laboratory confirmation. Rotavirus is a leading cause of severe gastroenteritis in children worldwide, characterized by sudden onset of watery diarrhea, vomiting, and fever.

**Why the Correct Answer is Right**
The correct answer is stool antigen detection, which involves detecting viral antigens present in stool samples using enzyme immunoassay (EIA) or immunochromatographic assays. These tests are highly specific and sensitive for detecting rotavirus, especially in the acute phase of infection. This method is preferred over other diagnostic modalities due to its rapid turnaround time, ease of use, and minimal sample preparation requirements.

**Why Each Wrong Option is Incorrect**
**Option A:** Stool culture is not the preferred method for diagnosing rotavirus infection due to its time-consuming nature (typically requiring 3-5 days for results) and the need for specialized laboratory equipment.

**Option B:** Stool PCR (polymerase chain reaction) is a highly sensitive and specific method for detecting rotavirus, but it is more expensive and requires specialized equipment, making it less suitable for routine diagnosis.

**Option C:** Endoscopy is not a suitable diagnostic modality for rotavirus diarrhea, as it is an invasive procedure that is more relevant for diagnosing conditions like inflammatory bowel disease or gastrointestinal bleeding.

**Clinical Pearl / High-Yield Fact**
Stool antigen detection kits for rotavirus are widely available and can provide results within 15-30 minutes, making them an ideal choice for diagnosing rotavirus diarrhea in pediatric patients.

**Correct Answer:** B. Stool PCR (polymerase chain reaction)