Which of the following forms the lateral boundary of the anatomical snuff box?

Explanation

## Correct Answer: A. Extensor pollicis brevis and abductor pollicis longus The anatomical snuff box (tabatière anatomique) is a triangular depression on the dorsal aspect of the hand at the base of the thumb, clinically significant for palpating the radial artery and detecting scaphoid fractures. Its boundaries are defined by three tendons: the **lateral (radial) boundary** is formed by two tendons—the **abductor pollicis longus (APL)** and **extensor pollicis brevis (EPB)**—which run together in the first dorsal compartment of the wrist. The medial boundary is formed by the extensor pollicis longus (EPL), and the floor contains the scaphoid and trapezium bones. The APL and EPB are both innervated by the posterior interosseous nerve (PIN, branch of radial nerve) and originate from the posterior forearm. The EPB lies deep to APL but both are palpable as a ridge when the thumb is extended and abducted. This anatomical landmark is crucial in Indian clinical practice for examining radial pulse (radial artery runs through the snuff box) and assessing scaphoid tenderness in suspected fractures—a common occupational injury in manual laborers. The key discriminator is recognizing that the **lateral boundary requires both APL and EPB together**, not EPL or abductor pollicis brevis (ABP, an intrinsic hand muscle). ## Why the other options are wrong **B. Extensor pollicis longus and abductor pollicis brevis** — This is wrong because EPL forms the **medial (ulnar) boundary**, not the lateral boundary, of the snuff box. ABP is an intrinsic hand muscle (thenar eminence) that does not contribute to snuff box boundaries at all. This option conflates the medial boundary (EPL) with a muscle that has no anatomical relationship to the snuff box, representing a common confusion between extrinsic and intrinsic thumb muscles. **C. Abductor pollicis longus and abductor pollicis brevis** — This is wrong because while APL correctly forms part of the lateral boundary, ABP (an intrinsic thenar muscle) does not. ABP is located in the palm and is not palpable on the dorsum of the hand. This option traps students who recognize APL's role but incorrectly assume that 'abductor' muscles must work together, ignoring the anatomical distinction between extrinsic forearm muscles and intrinsic hand muscles. **D. Extensor pollicis longus and extensor pollicis brevis** — This is wrong because EPL forms the **medial boundary**, not the lateral boundary. While EPB is correct for the lateral boundary, pairing it with EPL creates both boundaries, which is not what the question asks. This option is a classic NBE trap that tests whether students understand the specific anatomical position of each tendon and can distinguish lateral from medial boundaries. ## High-Yield Facts - **Lateral boundary of anatomical snuff box**: formed by APL and EPB tendons running together in the first dorsal compartment - **Medial boundary**: formed by EPL tendon (lies deeper, forms the posteromedial ridge) - **Floor of snuff box**: scaphoid and trapezium bones—site of radial artery palpation and scaphoid fracture tenderness assessment - **Innervation**: APL and EPB supplied by **posterior interosseous nerve (PIN)**, a branch of the radial nerve - **Clinical significance**: radial artery runs through the snuff box; tenderness here suggests scaphoid fracture (common in fall on outstretched hand—FOOSH injury) - **Intrinsic vs extrinsic**: APL and EPB are extrinsic forearm muscles; ABP is intrinsic thenar muscle—never part of snuff box boundaries ## Mnemonics **APL-EPB = Lateral (Radial) Snuff Box** **A**bductor **P**ollicis **L**ongus and **E**xtensor **P**ollicis **B**revis form the **L**ateral boundary. Both start with the same letters (APL, EPB) and both are extrinsic forearm muscles in the first dorsal compartment. Remember: APL is more superficial, EPB lies deep to it. **EPL = Medial (Ulnar) Snuff Box** **E**xtensor **P**ollicis **L**ongus forms the medial boundary. It's the deepest of the three thumb extensors and runs through the third dorsal compartment at the wrist, then curves around the dorsal tubercle of radius to reach the thumb IP joint. ## NBE Trap NBE pairs "extensor pollicis longus" with "abductor pollicis brevis" (Option B) to trap students who confuse the medial boundary (EPL) with the lateral boundary, and who conflate extrinsic forearm muscles with intrinsic hand muscles. Similarly, Option D lists both EPL and EPB to test whether students can distinguish which boundary each forms. ## Clinical Pearl In Indian emergency departments, the snuff box is routinely palpated in suspected scaphoid fractures (very common in manual workers and motorcycle accident victims). Tenderness in the snuff box + pain on axial loading of the thumb = high suspicion for scaphoid fracture, even if initial X-rays are negative. The radial artery pulse is also felt here when assessing hand perfusion before vascular surgery. _Reference: Cunningham's Manual of Practical Anatomy (Upper Limb), or Bailey & Love's Short Practice of Surgery (Ch. 65, Hand Surgery); also see Gray's Anatomy for detailed dorsal hand compartments_