**Core Concept**
Modified radical neck dissection (MRND) is a surgical procedure that involves the removal of lymph nodes and other structures in the neck, while preserving certain vital nerves and muscles. This procedure is often used to treat cancers of the head and neck, such as thyroid, parotid, and laryngeal cancers.

**Why the Correct Answer is Right**
In a MRND, the sternocleidomastoid muscle (SCM) and the internal jugular vein are preserved, whereas in a traditional radical neck dissection (RND), these structures are removed. The SCM is an important muscle for neck movement and rotation, and the internal jugular vein is a vital blood vessel that returns deoxygenated blood from the head and neck to the heart. By preserving these structures, MRND aims to minimize the functional and cosmetic consequences of the surgery.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify which structures are preserved in a MRND. While it is true that the accessory nerve is sometimes preserved, this option is too vague to be correct.
**Option B:** This option is incorrect because it specifically mentions the removal of the internal jugular vein, which is not characteristic of a MRND. In a MRND, the internal jugular vein is preserved.
**Option C:** This option is incorrect because it does not specify which nerves are preserved in a MRND. While it is true that some nerves are preserved, this option is too vague to be correct.

**Clinical Pearl / High-Yield Fact**
When performing a MRND, surgeons must carefully identify and preserve the accessory nerve, which is responsible for innervating the sternocleidomastoid and trapezius muscles. Damage to this nerve can result in significant weakness and disability.

**Correct Answer:** C. Preservation of the sternocleidomastoid muscle and internal jugular vein.