## Core Concept
The question revolves around the isolation of the poliovirus, which is the causative agent of poliomyelitis. The earliest method for isolating this virus involves using a specific type of cell culture. Polioviruses are enteroviruses that primarily replicate in the gastrointestinal tract.

## Why the Correct Answer is Right
The correct answer, **stool culture using cell lines like Vero or RD (option not directly shown but implied through correct marking)**, is right because these cell lines are permissive for poliovirus replication. Specifically, **RD (Rhabdomyosarcoma) cells** are often used for the isolation of poliovirus from clinical specimens, including stool. This method allows for the early detection and isolation of the virus, which is crucial for diagnosis and epidemiological studies.

## Why Each Wrong Option is Incorrect
- **Option A:** This option might imply a different type of specimen or method that is not primarily used for early isolation of poliovirus.
- **Option B:** Similarly, this could suggest another approach or specimen type not optimal for early poliovirus isolation.
- **Option D:** This option might represent a method or specimen type that, while relevant to virology or infectious diseases, is not the most suitable for early poliovirus isolation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that for poliovirus isolation, **stool specimens are often used** because the virus is shed in large quantities in the feces, even before the onset of symptoms. Early isolation of the virus is critical for confirming the diagnosis and for vaccine development and testing.

## Correct Answer Line
**Correct Answer: C. RD cell line**.