**Question:** Function of external intercostal -

A. Facilitating lung expansion
B. Controlling diaphragm movement
C. Involved in breathing during sleep
D. Both A and B

**Core Concept:**
The external intercostal muscles are a group of intercostal muscles located between the ribs. These muscles play a crucial role in breathing, specifically in the expansion of the thoracic cavity, which is necessary for efficient lung function. The external intercostal muscles help to move the ribcage outwards and downwards, allowing for increased lung volume and better oxygenation.

**Why the Correct Answer is Right:**
The correct answer is "D. Both A and B". The external intercostal muscles are primarily involved in facilitating lung expansion (option A) by moving the ribcage outwards and downwards, which increases the volume of the thoracic cavity. Additionally, they also control the movement of the diaphragm (option B), another key muscle involved in breathing.

**Why Each Wrong Option is Incorrect:**

A. Facilitating lung expansion (Option A) is only partially correct, as it fails to account for the external intercostal muscles' control of diaphragm movement as well.

B. Controlling diaphragm movement (Option B) is also only partially correct, as it does not address the function of the external intercostal muscles in facilitating lung expansion.

C. Controlling breathing during sleep (Option C) is not a relevant function of the external intercostal muscles. Their primary role is to facilitate lung expansion and control diaphragm movement, not to regulate breathing during sleep.

**Clinical Pearl:**
The external intercostal muscles are essential components of the respiratory system, working together with the diaphragm and internal intercostal muscles to ensure efficient lung expansion and oxygenation. A thorough understanding of the functions of these muscles is crucial for medical professionals to accurately diagnose and treat respiratory conditions in patients.