What is the primary clinical sign observed in a patient with a ruptured FDP?

The primary clinical sign observed in a patient with a ruptured flexor digitorum profundus (FDP) tendon is indeed the inability to flex at the proximal interphalangeal (PIP) joint. This specific injury typically occurs when there is a sudden load on the finger, often seen in sports. When the FDP tendon ruptures, it impairs the muscle's ability to effectively flex the distal phalanx through the PIP joint, leading to a characteristic inability to bend the affected finger at that joint while the metacarpophalangeal joint can still flex. This results in a "swan-neck" appearance of the finger, as the PIP joint is extended while the finger cannot flex at that joint, highlighting the functional impairment due to the tendon injury.

The other options, while they describe various symptoms that may arise from different hand or finger injuries, do not specifically align with the clinical presentation associated with a ruptured FDP. Pain on wrist extension relates more to wrist injuries or conditions affecting other structures like ligaments or tendons. Swelling at the base of the thumb is typically indicative of problems related to the thumb's tendons or ligaments, such as De Quervain's tenosynovitis.