A patient comes in complaining that she cannot flex her hand at the joint marked by a black arrow in the diagram. Which of the following muscles appear(s) to be paralyzed on fuher examination of her finger?
The muscles responsible for flexing the MCP joints are the flexor digitorum superficialis (FDS) and the lumbricals. The FDS inserts on the middle phalanx and is primarily responsible for flexion at the MCP and proximal interphalangeal (PIP) joints. The lumbricals, which arise from the tendons of the flexor digitorum profundus (FDP), help in flexing the MCP and extending the IP joints. However, if the black arrow is specifically at the MCP joint, the main muscle involved is the FDS. The FDP mainly acts on the PIP and distal interphalangeal (DIP) joints, so if the issue was there, FDP would be affected. The intrinsic muscles like the lumbricals and interossei also contribute but are more involved in extension at the IP joints. So if the patient can't flex the MCP, the FDS is likely paralyzed. Let's check the options. The correct answer should be the flexor digitorum superficialis. The other options might include the FDP, which affects lower joints, or the interossei, which are more for extension. Also, the flexor carpi radialis deals with wrist flexion, not finger MCP.
**Core Concept**
This question tests knowledge of finger flexion mechanics, specifically the muscles responsible for flexing the metacarpophalangeal (MCP) joint. The flexor digitorum superficialis (FDS) is the primary muscle for MCP joint flexion, while the flexor digitorum profundus (FDP) acts on the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints.
**Why the Correct Answer is Right**
The **flexor digitorum superficialis** (FDS) originates from the medial epicondyle of the humerus and inserts into the middle phalanges of fingers 2–5. It flexes the MCP and PIP joints via its tendons, which pass through the fibrous sheaths of the fingers. If the FDS is paralyzed, MCP joint flexion is impaired, but PIP/DIP flexion remains intact (since the FDP is unaffected). The black arrow likely marks the MCP joint, making FDS the correct answer.
**Why Each Wrong Option is Incorrect**
**Option A:** Flexor carpi radialis acts solely on the wrist, not finger joints.
**Option B:** Flexor digitorum profundus (FDP) flexes PIP/DIP joints, not MCP joints.
**Option C:** Lumbricals and interossei are intrinsic hand muscles that assist in IP joint extension, not MCP flexion.
**Clinical Pearl**
Remember: **“Superficial for MCP, Deep for PIP and DIP”**. The FDS (superficial) acts on the MCP and PIP, while the F