**Core Concept**
The posterior belly of the digastric muscle is a superficial structure in the neck, and knowing its relationship with deeper structures is essential for understanding neck anatomy. The posterior belly of the digastric muscle is a part of the digastric muscle, a muscle of facial expression that plays a role in opening the mouth.

**Why the Correct Answer is Right**
The structures lying deep to the posterior belly of the digastric muscle are the hypoglossal nerve, hyoglossus muscle, and the occipital artery. The hypoglossal nerve is a cranial nerve that innervates the tongue muscles, and it is located deep to the posterior belly of the digastric muscle. The hyoglossus muscle is also a muscle of the tongue that helps in tongue movement, and it lies deep to the posterior belly of the digastric muscle. The occipital artery is a branch of the external carotid artery that supplies blood to the scalp, and it is located deep to the posterior belly of the digastric muscle. On the other hand, the retromandibular vein is a superficial structure in the face that does not lie deep to the posterior belly of the digastric muscle.

**Why Each Wrong Option is Incorrect**
**Option A:** The retromandibular vein is a superficial structure that lies anterior to the posterior belly of the digastric muscle, not deep to it.

**Option B:** The hypoglossal nerve, on the other hand, lies deep to the posterior belly of the digastric muscle.

**Option C:** The hyoglossus muscle also lies deep to the posterior belly of the digastric muscle.

**Option D:** The occipital artery is a deeper structure that lies deep to the posterior belly of the digastric muscle.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the posterior belly of the digastric muscle is a superficial structure in the neck, and knowing its relationship with deeper structures can help in understanding neck anatomy and preventing misidentification of structures during surgical procedures.

**✓ Correct Answer: A. Retromandibular vein**