## Core Concept
The pin index system is a safety feature designed to prevent incorrect cylinder attachments in medical settings. It ensures that the correct gas cylinder is connected to the anesthesia machine or other medical equipment. This system is crucial for patient safety, as incorrect gas connections can lead to serious complications or even death.

## Why the Correct Answer is Right
The pin index system for oxygen cylinders is specifically designed with unique configurations of pins and holes on the yoke of the anesthesia machine and the cylinder head. The correct configuration for an oxygen cylinder is **2, 5**, which corresponds to specific positions of pins on the yoke of the anesthesia machine. This configuration prevents oxygen cylinders from being mistakenly connected to equipment meant for other gases.

## Why Each Wrong Option is Incorrect
* **Option A:** 1, 3 - This configuration does not match the pin index for oxygen cylinders, which could lead to incorrect cylinder attachment.
* **Option B:** 3, 5 - Although one of the numbers matches, the combination is not correct for oxygen, potentially leading to incorrect cylinder use.
* **Option D:** 1, 5 - Similarly, this combination does not match the specified pin index for oxygen cylinders.

## Clinical Pearl / High-Yield Fact
A key clinical pearl to remember is that the pin index system varies by country, but for oxygen in many countries, including India and following international standards, the pin index is **2, 5**. It's essential to familiarize oneself with the local standards and the specific pin indexes for different gases to ensure patient safety.

## Correct Answer: C. 2, 5