A 32-year-old male presents with a stab wound to the mid-arm that damages the radial nerve as it exits the spiral groove. On examination, all of the following clinical findings are expected EXCEPT:

Explanation

## Radial Nerve Injury: Clinical Presentation ### Anatomical Context **Key Point:** A mid-arm radial nerve injury (at or distal to the spiral groove) results in loss of all radial nerve motor and sensory functions distal to the lesion. ### Expected Clinical Findings After Radial Nerve Injury | Finding | Mechanism | Status in This Case | |---|---|---| | **Wrist drop** | Loss of ECRL, ECRB, ECU → inability to extend wrist | ✓ LOST | | **Thumb extension loss** | Loss of EPB, EIP → cannot extend IP joint of thumb | ✓ LOST | | **Forearm pronation** | Pronator teres (median/AIN) and pronator quadratus (AIN) intact | ✓ PRESERVED | | **Dorsal 1st web space sensation** | Superficial radial nerve (sensory branch) | ✓ LOST | | **Thumb IP flexion** | Flexor pollicis longus (AIN — median nerve) | ✓ PRESERVED | ### Why Option 3 is CORRECT (the exception) **High-Yield:** Flexion at the interphalangeal (IP) joint of the thumb is performed by the **flexor pollicis longus (FPL)**, which is innervated by the **anterior interosseous nerve (AIN)**, a branch of the **median nerve**—not the radial nerve. Therefore, IP flexion of the thumb is **preserved** after radial nerve injury. **Clinical Pearl:** Students often confuse thumb extension (radial nerve → EPB, EIP) with thumb flexion (median nerve → FPL). A radial nerve injury causes loss of thumb extension but preserves thumb flexion. ### Verification of Other Options 1. **Option 0 (Expected):** Wrist extension is lost due to paralysis of ECRL, ECRB, and ECU. 2. **Option 1 (Expected):** Pronation is preserved because both pronator muscles (pronator teres and pronator quadratus) are innervated by the median nerve (AIN). 3. **Option 2 (Expected):** Sensory loss over the dorsal 1st web space occurs because the superficial radial nerve (terminal sensory branch) is damaged. ## Mnemonic: **LOAF** and Thumb Movements **LOAF** (Median nerve — lateral 3.5 fingers): - **L**umbrical (digits 1–2) - **O**pponens pollicis - **A**bductor pollicis brevis - **F**lexor pollicis brevis **FPL** (Anterior interosseous nerve — median) = Flexor pollicis longus (thumb IP flexion). **EPB + EIP** (Radial nerve) = Thumb extension.