## **Core Concept**
The Pin Index Safety System (PISS) or PIN index is a safety feature designed to prevent incorrect connection of medical gas cylinders. It ensures that a specific gas cylinder can only be connected to equipment designed for that gas, thereby reducing the risk of medical gas mix-ups.

## **Why the Correct Answer is Right**
The PIN index for nitrous oxide cylinders is specifically designed to match with the corresponding index on medical equipment, ensuring that only the correct gas is used. The correct PIN index for nitrous oxide is **3 and 5**. This configuration is standardized to prevent accidental misconnection with other gas cylinders.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 1 and 5. This is incorrect because it does not match the standardized PIN index for nitrous oxide.
- **Option B:** 2 and 5. This is incorrect because it also does not match the standardized PIN index for nitrous oxide.
- **Option D:** 6. This option is incomplete as PIN indexes are typically described with two numbers.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the PIN index system helps prevent gas mix-ups, which can be life-threatening. For example, the PIN indexes for common medical gases are as follows: Oxygen (2, 5), Nitrous Oxide (3, 5), and Air (1, 5). Familiarizing oneself with these can significantly enhance patient safety.

## **Correct Answer:** . 3 and 5.