## **Core Concept**
The pin index system is a safety feature designed for medical gas cylinders to prevent incorrect connection of medical gases. It involves specific configurations of pins on the yoke of anesthesia machines that correspond to holes on the cylinder heads, ensuring that only the correct gas cylinder can be connected.

## **Why the Correct Answer is Right**
The correct pin index for an air cylinder is **(3) and (5)**. This specific configuration is standardized to prevent incorrect cylinder attachments, which could lead to serious clinical errors. The pin index system is crucial for patient safety, as incorrect gas connections could result in hypoxia or administration of an incorrect gas mixture.

## **Why Each Wrong Option is Incorrect**
- **Option A:** (1) and (5) - This configuration is incorrect for air; it's actually the pin index for Oxygen.
- **Option B:** (2) and (5) - This is not the correct pin index for air or any other commonly used medical gas.
- **Option D:** (1) and (6) - This is incorrect for air; it corresponds to a different gas.

## **Clinical Pearl / High-Yield Fact**
A crucial clinical pearl is to memorize the pin indexes for commonly used gases: Oxygen (2, 5), Nitrous Oxide (3, 5), and Air (1, 5) is incorrect as per the question context, emphasizing the need to recall that Air's correct index is actually (3) and (5). Always verify cylinder connections to prevent accidents.

## **Correct Answer:** .