A patient woke up with difficulty in extending fingers, can make a grip and hold a pen. Dorsiflexon was normal. Sensory and motor examination was normal. Nerve most commonly involved:
## **Core Concept**
The question presents a clinical scenario suggestive of a specific type of nerve injury affecting hand function. The key symptoms include difficulty in extending fingers, preserved grip strength, normal dorsiflexion, and normal sensory and motor examination otherwise. This pattern of symptoms points towards a lesion affecting a specific nerve that controls finger extension.
## **Why the Correct Answer is Right**
The correct answer, **Posterior Interosseous Nerve (PIN)**, is a branch of the radial nerve. The PIN primarily supplies the muscles responsible for finger extension (extensor digitorum, extensor indicis, and extensor digiti minimi) and thumb extension (extensor pollicis brevis and extensor pollicis longus), but not the muscles responsible for wrist extension (extensor carpi radialis brevis and extensor carpi radialis longus). This selective involvement explains why the patient can make a grip (flexor muscles are intact) and has normal dorsiflexion (wrist extension is preserved due to sparing of the radial nerve branches that control wrist extensors). The sensory examination is normal because the PIN is purely motor.
## **Why Each Wrong Option is Incorrect**
- **Option A:** The **Median Nerve** primarily controls thumb opposition, flexion of the thumb, index, and middle fingers, and provides sensation to the palmar surface of the thumb, index, middle finger, and the radial half of the ring finger. The symptoms described do not align with median nerve injury.
- **Option B:** The **Anterior Interosseous Nerve (AIN)**, a branch of the median nerve, supplies muscles involved in thumb and finger flexion (flexor pollicis longus, pronator quadratus, and the radial part of flexor digitorum profundus). Injury to AIN would not cause difficulty in extending fingers.
- **Option D:** The **Ulnar Nerve** controls finger spreading and adduction (interossei and lumbricals to the ring and little fingers), and provides sensation to the little finger and the ulnar half of the ring finger. The clinical presentation does not match ulnar nerve injury.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the **Posterior Interosseous Nerve (PIN) syndrome**, also known as radial tunnel syndrome when compressive, typically presents with weakness in finger extension without sensory deficits. This condition can be confused with other radial nerve pathologies but is distinguished by the sparing of wrist extension and sensation.
## **Correct Answer:** . **Posterior Interosseous Nerve**