**Question:** Painful arc syndrome is caused by the tear of

A. the flexor digitorum superficialis muscle
B. the flexor digitorum profundus muscle
C. the flexor digitorum brevis muscle
D. the flexor retinaculum

**Correct Answer:** D. the flexor retinaculum

**Core Concept:** Painful arc syndrome is a clinical entity characterized by pain in the flexor aspect of the proximal interphalangeal joint of the thumb and fingers. It is caused by the tear of the flexor retinaculum, which is a thick band of fibrous tissue that anchors the flexor tendons to the bone at the proximal interphalangeal joint.

**Why the Correct Answer is Right:** The flexor retinaculum serves as a barrier to prevent the flexor tendons from rubbing against the bone when the joint is in motion. When this barrier is torn or damaged, the flexor tendons can rub directly against the bone, causing pain and inflammation in the surrounding tissues.

**Why Each Wrong Option is Incorrect:**

A. The flexor digitorum superficialis muscle and the flexor digitorum profundus muscle are muscles that move the fingers, not cause painful arc syndrome. They are involved in finger flexion and abduction, respectively.

B. The flexor digitorum brevis muscle is a small muscle on the palmar aspect of the thumb and fingers, not involved in causing painful arc syndrome.

C. The flexor digitorum brevis muscle helps in flexing the thumb and fingers; it is not responsible for causing painful arc syndrome.

**Clinical Pearl:** Painful arc syndrome is a clinical entity that should be considered in patients presenting with pain and tenderness over the flexor aspect of the proximal interphalangeal joint of the thumb and fingers. Early diagnosis and treatment of this condition can prevent long-term complications such as tendon adhesion and contracture.