PERIPHERAL NERVE ANATOMY
Clinical MCQs: Radial, Ulnar & Median Nerves
ANSWER KEY WITH EXPLANATIONS
SECTION A: STANDARD LEVEL (Questions 1-10)
Question 1
Answer: B
Radial nerve in the spiral groove. The classic presentation of “Saturday night palsy” occurs when the radial nerve is compressed against the humerus in the spiral groove during prolonged pressure. This causes wrist drop, finger drop, and loss of thumb extension. Sensory loss over the first dorsal web space is pathognomonic for radial nerve injury.
Key Points:
- Wrist drop is the hallmark sign of radial nerve palsy
- First dorsal web space sensory loss distinguishes radial from other nerve injuries
- Triceps function preserved if injury is at spiral groove
Question 2
Answer: B
Median nerve – anterior interosseous branch. The anterior interosseous nerve (AIN) is a pure motor branch supplying flexor pollicis longus (FPL) and the lateral half of flexor digitorum profundus (FDP). Injury prevents flexion of the thumb IP joint and index finger DIP joint, making the “OK” sign impossible. There is no sensory loss as this is purely a motor branch.
Key Points:
- AIN is purely motor – no sensory deficit expected
- Classic triad: FPL, FDP to index/middle, pronator quadratus
- Unable to make “OK” sign is diagnostic for AIN palsy
Question 3
Answer: B
Carpal tunnel syndrome. CTS involves median nerve compression at the wrist and is the most common entrapment neuropathy. Classic features include thenar atrophy, weak thumb opposition/abduction, positive Tinel’s sign at wrist, and sensory loss over lateral 3½ digits palmarly.
Key Points:
- Most common nerve entrapment syndrome
- Positive provocative tests: Tinel’s and Phalen’s
- Thenar eminence sensation preserved (palmar cutaneous branch outside tunnel)
Question 4
Answer: C
Anterior interosseous nerve. Supracondylar fractures can injure the AIN, which innervates flexor pollicis longus (responsible for thumb IP flexion). Unlike main median nerve injury, there is no sensory loss since AIN is purely motor.
Key Points:
- AIN vulnerable in supracondylar fractures
- Pure motor deficit without sensory loss
- Loss of thumb IP flexion is key finding
Question 5
Answer: B
Ulnar nerve at elbow. Ulnar nerve injury at the cubital tunnel causes clawing of ring and little fingers, weak interossei, positive Froment’s sign, and sensory loss over medial 1½ fingers. This is the second most common entrapment neuropathy.
Key Points:
- Clawing more pronounced in ring and little fingers
- Froment’s sign indicates weak adductor pollicis
- “Claw more in low lesion” – distal lesions show more clawing
Question 6
Answer: D
Radial nerve. The radial nerve travels in the spiral groove of the mid-shaft humerus, making it the most vulnerable structure in humeral shaft fractures. Results in wrist drop, inability to extend thumb and fingers.
Key Points:
- Radial nerve most commonly injured in humeral shaft fractures
- Runs directly in contact with bone in spiral groove
- Classic triad: wrist drop, finger drop, thumb extension loss
Question 7
Answer: C
Recurrent motor branch of median nerve. The recurrent (thenar) motor branch has variable anatomy and arises distal to the carpal tunnel to supply thenar muscles. Can be injured during carpal tunnel release surgery, leading to permanent thenar weakness.
Key Points:
- Variable anatomy: extraligamentous (50%), subligamentous (31%)
- Supplies: APB, opponens pollicis, superficial FPB
- Important medicolegal consideration
Question 8
Answer: B
Ulnar nerve. The ulnar nerve innervates adductor pollicis. Froment’s sign indicates adductor pollicis weakness. Preserved thumb opposition and intact thenar sensation confirm isolated ulnar nerve injury.
Key Points:
- Froment’s sign is pathognomonic for weak adductor pollicis
- Opposition preserved because APB (median nerve) is intact
- Sensory pattern confirms isolated ulnar injury
Question 9
Answer: B
Index and middle fingers; median nerve. FDP has dual innervation: median nerve (via AIN) supplies FDP to index/middle; ulnar nerve supplies ring/little. Isolated median nerve injury affects DIP flexion of lateral two digits only.
Key Points:
- FDP: Median (index/middle) vs Ulnar (ring/little)
- Remember: “Median = lateral 2, Ulnar = medial 2”
- AIN supplies median portion of FDP
Question 10
Answer: B
Acute carpal tunnel syndrome. Acute CTS secondary to compartment syndrome or hematoma post-fracture. This is a surgical emergency requiring urgent decompression to prevent permanent nerve damage.
Key Points:
- Acute CTS is a surgical emergency unlike chronic CTS
- Can occur post-trauma (fracture, crush injury)
- Requires urgent surgical decompression
SECTION B: ADVANCED LEVEL (Questions 11-20)
Question 11
Answer: B
Posterior interosseous nerve injury at arcade of Frohse. Preserved triceps indicates injury distal to spiral groove. Normal posterior forearm sensation rules out main radial nerve injury. PIN is purely motor. Arcade of Frohse is the most common site of PIN compression.
Key Points:
- PIN = pure motor (no sensory loss)
- Arcade of Frohse most common compression site
- Also called “radial tunnel syndrome”
Question 12
Answer: B
Median nerve lesion distal to pronator teres but proximal to carpal tunnel. FCR spared indicates lesion distal to pronator teres. Thenar involvement suggests proximal to carpal tunnel. No sensory loss because palmar cutaneous branch intact. Seen in uremic neuropathy.
Key Points:
- FCR spared = distal to pronator teres
- Thenar involvement = proximal to carpal tunnel
- Common in hemodialysis patients
Question 13
Answer: A
Gantzer’s muscle is intact. Classic AIN injury affects FPL, lateral FDP, and pronator quadratus. Gantzer’s muscle (accessory FPL/FDP head) present in 45-60% can have dual innervation. Pronation maintained by pronator teres.
Key Points:
- Gantzer’s muscle present in 45-60%
- Can receive anomalous innervation
- Pronator teres compensates for paralyzed PQ
Question 14
Answer: B
Ulnar nerve at Guyon’s canal (Zone 1). Handlebar palsy causes ulnar compression at Guyon’s canal. ABSENCE of clawing indicates FDP preserved (distal lesion). Zone 1 affects motor and sensory. Dorsal sensation preserved.
Key Points:
- No clawing because FDP intact
- Zone 1 = motor + sensory involvement
- Dorsal sensation preserved
Question 15
Answer: C
Marinacci communication. Marinacci (reverse Martin-Gruber) involves ulnar to median crossover. Proximal ulnar injury affects intrinsic hand muscles, but FDP to ring/little preserved because fibers traveled via median nerve. Rare (1-4%) but high-yield.
Key Points:
- Marinacci = ulnar to median crossover
- Very rare: 1-4% vs Martin-Gruber 15-20%
- Explains preserved FDP with proximal ulnar injury
Question 16
Answer: B
Ulnar nerve supplies FDP to index, middle, and ring fingers. Isolated middle finger DIP weakness with preserved index suggests anomalous innervation via Martin-Gruber anastomosis. FDP innervation shows variation in up to 30%.
Key Points:
- FDP innervation highly variable (up to 30%)
- Martin-Gruber can alter standard pattern
- Tests understanding of anatomical variations
Question 17
Answer: B
Anterior interosseous nerve syndrome. “Pseudo-ulnar claw” affects index/middle (not ring/little). FDP to index/middle paralyzed causes lumbricals to lose tension, fingers assume claw position. Intrinsic muscles remain functional. Common in diabetes.
Key Points:
- “Pseudo-ulnar claw” affects index/middle
- Mechanism: FDP paralysis → lumbrical dysfunction
- Intrinsic muscles intact
Question 18
Answer: E
Posterior interosseous nerve injury. Preserved triceps indicates distal to axilla. Preserved brachioradialis suggests distal to spiral groove. PIN (pure motor) causes wrist/finger drop. Sensory loss suggests complex injury pattern.
Key Points:
- Preserved triceps = distal to proximal arm
- Preserved BR = distal to spiral groove
- PIN = pure motor
Question 19
Answer: C
Isolated injury to recurrent motor branch of median nerve. Recurrent branch arises AFTER carpal tunnel with variable anatomy. Supplies thenar muscles. Isolated injury causes pure motor deficit without sensory loss or positive carpal tunnel tests.
Key Points:
- Recurrent branch arises DISTAL to carpal tunnel
- Negative Tinel’s/Phalen’s rules out CTS
- EMG shows isolated thenar denervation
Question 20
Answer: B
Pain with resisted forearm pronation. Pronator syndrome involves median nerve compression in proximal forearm. Key discriminator: pain with resisted pronation compresses nerve between PT heads. Palm sensation affected (unlike CTS). Activity-related, not nocturnal.
Key Points:
- Pain with resisted pronation is pathognomonic
- Palm sensation affected (key differentiator from CTS)
- Activity-related symptoms, not nocturnal
HIGH-YIELD SUMMARY
Anatomical Variations:
- Martin-Gruber (15-20%): Median to ulnar crossover
- Marinacci (1-4%): Ulnar to median crossover
- Gantzer’s muscle (45-60%): Accessory FPL/FDP
Localization Rules:
- Triceps preserved = distal to axilla
- BR preserved = distal to spiral groove
- FCR preserved = distal to pronator teres
- No sensory loss = pure motor branch
Classic Signs:
- Froment’s: Weak adductor pollicis (ulnar)
- OK sign impossible: AIN palsy
- Claw hand: Ulnar (ring/little)
- Pseudo-ulnar claw: AIN (index/middle)
SCORING GUIDE
18-20 correct:
Excellent – Ready for exams
15-17 correct:
Very Good – Review advanced concepts
12-14 correct:
Good – Focus on variations
9-11 correct:
Fair – Strengthen basics
Below 9:
Needs significant review