**Question:** Colour of oxygen cylinder is

A. Red
B. Blue
C. Green
D. Purple

**Core Concept:**
The colour of oxygen cylinders is used as a visual cue to differentiate between medical gases, particularly oxygen and air. These colours are standardized globally to ensure safe handling and administration of medical gases in healthcare settings.

**Why the Correct Answer is Right:**
The correct answer is **D. Purple**, as it represents the colour for medical oxygen. Purple is chosen for oxygen because it contrasts well with the blue colour of air cylinders and other background colours commonly used in healthcare settings. This helps healthcare professionals quickly identify the gas content without needing to check the label, ensuring safer and faster gas delivery during medical procedures.

**Why Each Wrong Option is Incorrect:**
A. **Red (Option A)** is incorrect because red is used for medical air cylinders. Air contains approximately 78% nitrogen, 21% oxygen, and 1% other gases, making it unsuitable for oxygen therapy.

B. **Blue (Option B)** is incorrect because blue is used for medical air cylinders. As mentioned earlier, air is not suitable for oxygen therapy due to its composition.

C. **Green (Option C)** is incorrect because green is not a standardized colour for oxygen cylinders. The use of green might be found in some regional or local practices, but globally, the correct colour for oxygen is purple.

**Clinical Pearl:**
Understanding the colour code for medical gases is crucial for healthcare professionals, especially nurses, respiratory therapists, and physicians. By recognizing the correct gas colour, they can quickly identify the content of the cylinder without checking the label, ensuring patient safety during medical procedures.

**Correct Answer:** D. Purple