Correct Answer: A. Flexion
The muscle marked as A is the flexor digitorum superficialis (FDS) or flexor digitorum profundus (FDP), both of which are intrinsic flexors of the metacarpophalangeal (MCP) joint. At the MCP joint, these muscles produce flexion because their tendons pass volar (palmar) to the axis of rotation of the MCP joint. The axis of rotation at the MCP joint runs transversely through the metacarpal head. Any muscle with a moment arm on the volar side of this axis will produce flexion. The flexor digitorum muscles originate from the forearm, pass through the carpal tunnel, and insert on the phalanges distal to the MCP joint. When they contract, they pull the phalanges in a volar direction, causing the fingers to curl into flexion—the primary functional movement for grip and precision pinch in Indian clinical practice. This is distinct from the intrinsic hand muscles (lumbricals and interossei), which have a more complex action at the MCP joint due to their dorsal insertion on the extensor apparatus.
Why the other options are wrong
B. Extension — Extension at the MCP joint is produced by the extensor digitorum communis (EDC) and extensor indicis proprius, whose tendons pass dorsal to the MCP joint axis. The muscle marked A passes volar to the axis, making it a flexor, not an extensor. This is a common NBE trap—confusing the anatomical course of flexor vs. extensor tendons in the hand. C. Adduction — Adduction at the MCP joint is produced by the palmar interossei (PAD—Palmar ADduct), which have insertions on the medial side of the extensor apparatus. The muscle A is a long flexor of the forearm, not an intrinsic hand muscle, so it has no adduction function at the MCP joint. Adduction/abduction occur primarily at the MCP joint, not at the interphalangeal joints. D. Abduction — Abduction at the MCP joint is produced by the dorsal interossei (DAB—Dorsal ABduct), which spread the fingers apart. The muscle A is a flexor with a volar moment arm; it cannot produce abduction. This option tests whether students confuse the anatomical actions of extrinsic forearm muscles with intrinsic hand muscles.
High-Yield Facts
- Flexor digitorum superficialis and profundus pass volar to the MCP joint axis → produce flexion at MCP joint.
- Extensor digitorum communis passes dorsal to the MCP joint axis → produces extension at MCP joint.
- Lumbricals insert on the lateral bands of the extensor apparatus → produce MCP flexion AND IP extension (complex action).
- Palmar interossei (PAD) adduct fingers; dorsal interossei (DAB) abduct fingers at the MCP joint.
- In Indian hand surgery, flexor digitorum profundus is the primary muscle tested for grip strength and finger flexion in clinical examination.
Mnemonics
PAD & DAB for Intrinsic Hand Muscles PAD = Palmar interossei ADduct (bring fingers together). DAB = Dorsal interossei ABduct (spread fingers apart). Use this to remember which intrinsic muscles control adduction/abduction at the MCP joint—the extrinsic flexors/extensors do not. Volar = Flexion, Dorsal = Extension Any tendon passing volar (palmar) to the MCP joint axis produces flexion. Any tendon passing dorsal produces extension. This simple rule applies to all long muscles of the forearm acting on the hand.
NBE Trap
NBE commonly pairs forearm flexor muscles with intrinsic hand muscle actions (adduction/abduction) to trap students who confuse extrinsic and intrinsic hand anatomy. The correct answer requires knowing that flexor digitorum muscles act only on flexion/extension at the MCP joint, not on abduction/adduction.
Clinical Pearl
In Indian orthopedic and hand surgery practice, testing flexor digitorum profundus strength (by asking the patient to flex the distal interphalangeal joint while the MCP and PIP joints are held extended) is a routine bedside test for ulnar nerve integrity and grip function—essential after hand trauma or nerve injury repair.
_Reference: Bailey & Love's Short Practice of Surgery (Hand Surgery chapter); Clinically Oriented Anatomy by Moore (Upper Limb, Hand section)_