Stenosing tenosynovitis, commonly known as ‘trigger finger’, is characterized by painful catching or locking of the digit during flexion and extension. This mechanical impingement occurs strictly at the level of which specific flexor tendon pulley?

Explanation:
The correct option is a. Trigger finger is caused by a size mismatch between the swollen, inflamed flexor tendon (frequently developing a reactive nodule) and the tight fibro-osseous tunnel. This mechanical bottleneck occurs specifically at the A1 pulley, which overlies the metacarpophalangeal (MCP) joint. Surgical release of the A1 pulley reliably cures the condition. The A2 and A4 pulleys are critical biomechanical structures located over the proximal and middle phalanges, respectively; releasing them would cause a devastating mechanical 'bowstringing' of the flexor tendons and must be avoided. Reference: Green's Operative Hand Surgery.