A 43-year-old man is admitted to the hospital, having suffered a whiplash injury when his compact auto was struck from behind by a sports utility vehicle. MRI examination reveals some herniation of a disk in the cervical region. Physical examination reveals that the patient has lost elbow extension; there is absence of his triceps reflex and loss of extension of the metacarpophalangeal joints on the ipsilateral side. Which of the following spinal nerves is most likely affected?

## **Core Concept**
The question tests knowledge of spinal nerve root anatomy and its correlation with clinical presentation, particularly in the context of cervical disk herniation. The symptoms described point towards a specific nerve root injury affecting motor functions. The key is to identify which nerve root is responsible for the muscles and reflexes mentioned.

## **Why the Correct Answer is Right**
The patient's symptoms include loss of elbow extension, absence of the triceps reflex, and loss of extension of the metacarpophalangeal joints. These functions are primarily associated with the **C7 nerve root**. The C7 nerve root is responsible for:
- Elbow extension through its innervation of the triceps brachii.
- The triceps reflex.
- Extension of the metacarpophalangeal joints, primarily through innervation of the extensor digitorum muscle.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Typically, C6 nerve root injuries would affect elbow flexion (biceps brachii), forearm supination, and the biceps reflex, which doesn't match the symptoms described.
- **Option B:** C5 nerve root injuries usually affect shoulder abduction (deltoid), elbow flexion (biceps), and the biceps reflex, not matching the patient's symptoms.
- **Option D:** C8 nerve root injuries primarily affect finger extension (especially at the interphalangeal joints through the extensor digitorum communis and extensor indicis), finger flexion (flexor digitorum profundus), and sometimes the hand intrinsic muscles, which doesn't align well with the described deficits.

## **Clinical Pearl / High-Yield Fact**
A helpful clinical correlation to remember is the "**myotome**" and "**dermatome**" mapping for each spinal nerve root. Specifically, for the **C7 nerve root**:
- **Motor:** Elbow extension, wrist extension, and extension of the metacarpophalangeal joints.
- **Reflex:** Triceps reflex.

## **Correct Answer:** . C7.