**Core Concept:** The mandibular nerve (cranial nerve V3) is one of the three divisions of the trigeminal nerve. It supplies motor innervation to the muscles of mastication, except the masseter muscle, which is supplied by the radial nerve (cervical plexus).

**Why the Correct Answer is Right:** The mandibular nerve supplies motor innervation to the temporalis, masseter, and pterygoid muscles. The masseter muscle, however, is innervated by the radicular branch of the cervical plexus (cervical sympathetic chain), not the mandibular nerve. Therefore, the correct answer is **D** (the masseter muscle).

**Why Each Wrong Option is Incorrect:**

A. The temporalis muscle is correctly innervated by the mandibular nerve.
B. The pterygoid muscles are also supplied by the mandibular nerve.
C. The masseter muscle, as explained above, is innervated by the radicular branch of the cervical plexus.

**Clinical Pearl:** Understanding the anatomy of the trigeminal and cervical nerves is crucial for treating conditions like facial pain, facial paralysis, and temporomandibular joint disorders. A thorough neuroanatomy knowledge is essential for diagnosis and treatment planning.

**Correct Answer:** The correct answer is **D** (the masseter muscle), which is not supplied by the mandibular nerve but by the radicular branch of the cervical plexus.