**Core Concept**
Functional neck dissection is a surgical procedure that aims to remove metastatic lymph nodes while preserving the function of the neck and surrounding structures. This procedure is typically performed in patients with head and neck cancer, where the primary goal is to control the spread of cancer while maintaining the patient's quality of life.

**Why the Correct Answer is Right**
In a functional neck dissection, the sternocleidomastoid muscle (SCM), the internal jugular vein, and the spinal accessory nerve (SAN) are preserved. The SCM helps with neck rotation and forward flexion, while the internal jugular vein carries deoxygenated blood from the head and neck to the heart. The SAN, also known as the accessory nerve, innervates the sternocleidomastoid muscle and the trapezius muscle, which are essential for shoulder movement. Preserving these structures is crucial for maintaining the patient's neck and shoulder function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the submandibular gland is not typically preserved in a functional neck dissection, as it may contain metastatic lymph nodes.
**Option B:** This option is incorrect because the omohyoid muscle is often sacrificed during a functional neck dissection to ensure complete removal of the lymph nodes.
**Option C:** This option is incorrect because the hypoglossal nerve is often at risk during neck dissections, particularly if the surgeon is not careful, but it is not specifically preserved in a functional neck dissection.

**Clinical Pearl / High-Yield Fact**
In a functional neck dissection, the surgeon must carefully balance the need to remove metastatic lymph nodes with the need to preserve surrounding structures and maintain the patient's quality of life.

**Correct Answer: D. Sternocleidomastoid muscle, internal jugular vein, and spinal accessory nerve.**