**Core Concept**
The radial bursa is a synovial sheath surrounding the tendons of the flexor pollicis longus (FPL) and extensor pollicis longus (EPL) muscles. This bursa reduces friction between the tendons and the bone, allowing for smooth movement of the thumb.

**Why the Correct Answer is Right**
The infection in the radial bursa will most likely affect the tendon of the flexor pollicis longus (FPL) muscle. This is because the FPL tendon passes through the radial bursa, which has become infected. As the infection spreads, it will likely cause inflammation and damage to the FPL tendon, leading to symptoms such as pain, swelling, and limited thumb movement. The FPL muscle is responsible for flexion of the interphalangeal joint of the thumb, and damage to this tendon would impair this function.

**Why Each Wrong Option is Incorrect**
**Option A:** The flexor digitorum profundus (FDP) tendon is located in the ulnar bursa, which is separate from the radial bursa, and is therefore less likely to be affected by the infection.

**Option B:** The flexor digitorum superficialis (FDS) tendon is also located in the ulnar bursa and is not directly associated with the radial bursa.

**Option D:** The flexor carpi radialis (FCR) tendon is located in the wrist and is not associated with the radial bursa, making it an unlikely candidate for tendon damage in this scenario.

**Clinical Pearl / High-Yield Fact**
The radial bursa is a common site for tendonitis and bursitis, particularly in individuals who participate in activities that involve repetitive thumb motion, such as musicians or athletes. It's essential for healthcare professionals to be aware of the anatomy of the wrist and hand to accurately diagnose and treat conditions affecting the tendons and bursae.

**✓ Correct Answer: C. Flexor pollicis longus**