A 7-year-old boy presents to the emergency department after falling on an outstretched hand. X-ray shows an extension-type supracondylar fracture of the humerus. On examination, he is unable to make an 'OK' sign with his thumb and index finger. Which structure is most likely injured?

1. A

   Median nerve

2. B

   Ulnar nerve

3. C

   Radial nerve

4. D

   Brachial artery

Appeared in: INI-CET EXAM -2025

Explanation

• The inability to make an 'OK' sign is a pathognomonic finding for an injury to the anterior interosseous nerve (AIN).
• The AIN is a pure motor branch of the median nerve, making 'Median nerve' the correct parent structure.
• The AIN supplies the flexor pollicis longus and the flexor digitorum profundus to the index finger, the two muscles essential for the flexion required to make the 'OK' sign.
• The AIN is the most commonly injured nerve in extension-type supracondylar fractures due to its anatomical position anterior to the distal humerus.

Why Other Options Were Wrong

• Option B: Ulnar nerve injury affects the intrinsic muscles of the hand and sensation in the 4th and 5th fingers, causing a 'claw hand' deformity, not an 'OK' sign deficit.
• Option C: Radial nerve injury causes 'wrist drop' (inability to extend the wrist and fingers) because it supplies the posterior compartment of the forearm.
• Option D: Brachial artery injury is a vascular emergency that presents with signs of ischemia (pain, pallor, pulselessness), not an isolated motor deficit like the one described.

Related Visual

• Visual 1: Diagram - A clear illustration showing a patient attempting to make the 'OK' sign, highlighting the collapsed 'pinch' posture seen in AIN palsy.
• Visual 2: Anatomical Drawing - A diagram of the elbow joint showing the path of the median nerve and brachial artery in relation to a supracondylar fracture, illustrating the mechanism of injury.
• Visual 3: Table - A summary table comparing the signs of median, ulnar, and radial nerve injuries in the context of elbow trauma.

Clinical Relevance

• Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Neurological complications of supracondylar humerus fractures as background academic context rather than a clinical decision trigger.
• A thorough neurovascular examination is mandatory for any child presenting with an elbow fracture. The inability to make an 'OK' sign is a red flag for AIN/median nerve injury.
• Nurses must repeatedly assess and document the neurovascular status (pulses, capillary refill, sensation, motor function) of the affected limb, as swelling can lead to compartment syndrome, a surgical emergency.
• What if? If the patient had a weak or absent radial pulse in addition to the nerve palsy, this would indicate a combined brachial artery and median nerve injury, requiring immediate orthopedic consultation and possible surgical exploration to restore blood flow.

How to Approach the Question

• First, identify the key elements of the clinical scenario: a 7-year-old with a supracondylar fracture after a fall.
• Next, focus on the specific clinical sign provided: the inability to make an 'OK' sign.
• Recall or deduce the muscles and nerve responsible for the 'OK' sign action. This involves flexion of the thumb and index finger tips, controlled by the flexor pollicis longus and flexor digitorum profundus.
• Connect this action to the innervating nerve: the anterior interosseous nerve (AIN), which is a branch of the median nerve.
• Evaluate the options based on this connection. The 'OK' sign deficit points directly to the median nerve. Rule out the other nerves (ulnar, radial) and the artery by considering their different functions and signs of injury.

Concept Tested & Keywords

• Concept Tested: Neurological complications of supracondylar humerus fractures.
• Stem keywords: supracondylar fracture, humerus, outstretched hand, OK sign, thumb and index finger
• Lead-in keywords: most likely injured
• Clinical cues: The patient's inability to make an 'OK' sign is the key diagnostic clue, pointing directly to a specific nerve deficit.

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