The Correct Answer is Flexor Digitorum Profundus
The flexor digitorum profundus (FDP) is the only muscle that flexes the distal interphalangeal (DIP) joints of the medial four digits (index, middle, ring, and little fingers). 1
Anatomical Basis
The FDP is a deep flexor muscle of the forearm with a composite structure and dual nerve supply:
- Medial half: supplied by the ulnar nerve
- Lateral half: supplied by the anterior interosseous nerve (branch of median nerve)
Its unique function is flexion of the distal phalanges of the medial four digits 1, making it the sole muscle capable of DIP joint flexion in these fingers.
Why the Other Options Are Incorrect
Flexor pollicis longus (option a): This muscle flexes the distal phalanx of the thumb only, not the medial four digits 2
Flexor digitorum superficialis (option c): This muscle flexes the proximal interphalangeal (PIP) joints, not the DIP joints 3
Pronator quadratus (option d): This muscle pronates the forearm and has no role in finger flexion
Clinical Significance
The FDP's anatomical variations are clinically important 1:
- Extensive cleavage of the muscle belly can form a separate flexor indicis profundus
- Enlarged variants may compress the anterior interosseous nerve, causing compression neuropathy
- Mechanical connections between tendons limit independent finger movement, particularly between middle, ring, and little fingers 2
Answer: b. Flexor digitorum profundus