**Core Concept:** The obturator externus muscle is a muscle of the thigh found in the human body. It plays a significant role in hip joint stabilization and movement.

**Why the Correct Answer is Right:** The main action of the obturator externus muscle is to externally rotate the hip joint and abduct the thigh. This action is crucial for maintaining stability and facilitating movement in the hip joint. The muscle is involved in the flexion, abduction, and external rotation of the thigh, contributing to overall hip joint function.

**Why Each Wrong Option is Incorrect:**

A. **Incorrect:** The obturator externus muscle primarily acts on the hip joint, not the knee joint. Its primary function is not related to knee movement.

B. **Incorrect:** Although the rectus femoris muscle is part of the quadriceps group, it is responsible for knee flexion, not hip actions like obturator externus.

C. **Incorrect:** The gluteus medius muscle is involved in hip abduction and external rotation, yet its primary function is not related to hip rotation.

D. **Incorrect:** The tensor fasciae latae muscle is a gluteal muscle that helps in hip abduction, not hip rotation.

**Clinical Pearl / High-Yield Fact:** The obturator externus muscle is part of the obturator internus complex, which also includes the obturator internus and piriformis muscles. These muscles work together to control the movements of the hip joint, ensuring stability and facilitating smooth joint function.

**Correct Answer:** **C.** The gluteus medius muscle is primarily involved in hip abduction and external rotation, making it the correct answer to this question.