**Core Concept**
The posterior belly of the digastric muscle is a superficial muscle of the neck that originates from the digastric fossa of the mandible. The structures lying deep to this muscle are critical to identify during neck dissections or surgical procedures.

**Why the Correct Answer is Right**
The correct answer is the structure that does not lie deep to the posterior belly of the digastric muscle. The posterior belly of the digastric muscle lies deep to the parotid gland, the external carotid artery, and the facial nerve. These structures are important to identify during surgical procedures in the neck.

**Why Each Wrong Option is Incorrect**
**Option A:** The sternocleidomastoid muscle lies superficial to the posterior belly of the digastric muscle, not deep to it. This muscle is a key landmark in the neck and is often used for surgical approaches.
**Option B:** The omohyoid muscle lies deep to the posterior belly of the digastric muscle, making it a correct option. This muscle is a key landmark in the neck and is often used for surgical approaches.
**Option C:** The internal jugular vein lies deep to the posterior belly of the digastric muscle, making it a correct option. This vein is a critical structure in the neck and is often used for surgical approaches.

**Clinical Pearl / High-Yield Fact**
During neck dissections, it's essential to identify the posterior belly of the digastric muscle and the structures lying deep to it, including the external carotid artery and the facial nerve.

**Correct Answer: A. Sternocleidomastoid muscle**