**Core Concept:** Malignant otitis externa is a severe, invasive bacterial infection primarily affecting the external auditory canal. It can spread to adjacent structures, including the temporal bone and brain.

**Why the Correct Answer is Right:** In malignant otitis externa, the commonly affected nerve is the facial nerve (CN VII). The facial nerve is responsible for the function of facial muscles, including the muscles involved in smiling, crying, and closing the eye on the affected side. When the facial nerve is compromised, patients may present with facial weakness or palsy, which is a characteristic feature of this condition.

**Why Each Wrong Option is Incorrect:**

A. **Labyrinthine artery (CN VIII):** Although malignant otitis externa can involve the labyrinthine artery, this option is incorrect because the labyrinthine artery is a branch of the internal carotid artery supplying blood to the inner ear, not the facial nerve.

B. **Cochlear nerve (CN VIII):** Similar to option A, the cochlear nerve is a branch of the internal carotid artery and is not involved in facial nerve function or involvement in malignant otitis externa.

C. **Mastoid branch of the trigeminal nerve (CN V):** While the mastoid branch of the trigeminal nerve supplies sensory innervation to the ear, it is not the nerve primarily affected in malignant otitis externa. The facial nerve is the primary target in this condition.

D. **Oculomotor nerve (CN III):** The oculomotor nerve is responsible for eye movement, not facial nerve function. It is not involved in malignant otitis externa.

**Clinical Pearl:** Malignant otitis externa is a severe complication of Pseudomonas aeruginosa infection, often seen in immunocompromised patients, leading to progressive destruction of the temporal bone and surrounding structures. Treatment involves a combination of antibiotics, surgery, and in severe cases, a hearing aid or cochlear implant may be necessary to restore hearing and prevent complications.

**Correct Answer:** **Correct Answer: Facial nerve (CN VII)**

The correct answer is CN VII, also known as the facial nerve. Facial nerve involvement is essential in malignant otitis externa due to its role in controlling facial muscles responsible for facial expression, including smiling, crying, and closing the eye on the affected side.

In summary, malignant otitis externa is a life-threatening infection that involves the facial nerve, leading to facial palsy and other symptoms characteristic of this condition. Understanding the appropriate nerve involved in this situation is crucial for accurate diagnosis and management.