**Core Concept**
The sternocleidomastoid muscle is a complex structure with multiple blood supply sources, reflecting its embryological origin and anatomical location. The sternocleidomastoid muscle is supplied by branches from the occipital artery, the posterior branch of the superior thyroid artery, and the deep cervical artery.

**Why the Correct Answer is Right**
The sternocleidomastoid muscle receives its primary blood supply from the occipital artery, which is a branch of the external carotid artery. The posterior branch of the superior thyroid artery also contributes to its blood supply. However, the sternocleidomastoid muscle does not receive a direct blood supply from the costocervical trunk, which is a branch of the subclavian artery. The deep cervical artery, a branch of the costocervical trunk, primarily supplies the deep cervical muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** The occipital artery is indeed a correct source of blood supply for the sternocleidomastoid muscle. However, the question asks for an exception, and the occipital artery is one of the correct sources.
**Option B:** The posterior branch of the superior thyroid artery is also a correct source of blood supply for the sternocleidomastoid muscle.
**Option C:** The deep cervical artery is actually a correct source of blood supply for the sternocleidomastoid muscle, but it is not the primary source.

**Clinical Pearl / High-Yield Fact**
The sternocleidomastoid muscle is a key landmark for identifying the external jugular vein and the accessory nerve (cranial nerve XI), which runs through the muscle.

**Correct Answer:** D.