**Core Concept**
Pisiform is a small, pea-shaped sesamoid bone embedded within the flexor carpi ulnaris tendon, located on the ulnar (medial) side of the wrist. It plays a crucial role in the flexion of the wrist and forearm.

**Why the Correct Answer is Right**
The pisiform bone is a sesamoid bone, meaning it is embedded within a tendon to change the direction of muscle forces, allowing for more efficient flexion of the wrist. The flexor carpi ulnaris tendon, which contains the pisiform, attaches to the hamate bone in the distal row of the carpal bones. This unique structure enables the pisiform to contribute to the flexion of the wrist.

**Why Each Wrong Option is Incorrect**
**Option A:** The pisiform is not a sesamoid bone. (This statement is false; the pisiform is indeed a sesamoid bone embedded within the flexor carpi ulnaris tendon.)
**Option B:** The pisiform does not contribute to wrist flexion. (This statement is false; the pisiform, through its embedded position within the flexor carpi ulnaris tendon, does contribute to wrist flexion.)
**Option C:** The pisiform is located on the radial (lateral) side of the wrist. (This statement is false; the pisiform is actually located on the ulnar, or medial, side of the wrist.)
**Option D:** The pisiform is not a carpal bone. (This statement is false; the pisiform is actually a carpal bone, specifically a sesamoid bone embedded within the flexor carpi ulnaris tendon.)

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the pisiform bone is often the site of tendinitis or fractures due to its embedded position within the flexor carpi ulnaris tendon, which can be prone to strain and injury.

**Correct Answer: A. The pisiform is not a sesamoid bone.