A massive, neglected pyogenic infection of the flexor tendon sheath of the small (pinky) finger has a high propensity to spread proximally into the wrist and across to the thumb, creating a ‘horseshoe abscess’. This occurs because the small finger’s tendon sheath is directly anatomically continuous with which synovial space?

Explanation:
The correct option is d. In human anatomy, the flexor tendon sheaths of the index, middle, and ring fingers typically terminate blindly at the mid-palm. However, the flexor sheath of the small finger communicates directly and continuously with the ulnar bursa in the wrist. Similarly, the flexor pollicis longus (FPL) sheath of the thumb communicates directly with the radial bursa. In many individuals, the radial and ulnar bursae communicate with each other proximal to the carpal tunnel (Parona's space). Therefore, an infection in the pinky tracks rapidly down the ulnar bursa, crosses Parona's space, and travels up the radial bursa into the thumb, forming the devastating horseshoe abscess. Reference: Kanavel AB. Infections of the Hand.