**Question:** Toynbee's muscle is -

A. Rectus abdominis muscle
B. Sternocleidomastoid muscle
C. Latissimus dorsi muscle
D. External oblique muscle

**Core Concept:** Toynbee's muscle refers to a group of muscles that are involved in the breathing process and contribute to the intercostal muscles. These muscles are essential for chest expansion during inhalation and exhalation.

**Why the Correct Answer is Right:** The correct answer, D - External Oblique muscle, is part of the intercostal muscles that help expand the chest during inhalation. Intercostal muscles are located between the ribs and are responsible for the expansion of the thoracic cavity during respiration.

**Why Each Wrong Option is Incorrect:**

A - Rectus abdominis muscle (A) is incorrect because it is a muscle of the abdominal wall and does not contribute to chest expansion during respiration.

B - Sternocleidomastoid muscle (B) is incorrect as it is a muscle of the neck region and is not involved in chest expansion during respiration.

C - Latissimus dorsi muscle (C) is incorrect because it is a muscle of the back and does not contribute to chest expansion during respiration.

**Clinical Pearl / High-Yield Fact:** Understanding Toynbee's muscle and its role in respiration helps medical students grasp the anatomy and physiology of respiration, which is essential for assessing patients with respiratory issues and administering appropriate treatment.