Which muscle flexes and medially rotates the thigh?

The iliacus muscle is integral to both flexing and medially rotating the thigh. It is a part of the iliopsoas group, which also includes the psoas major, and collectively they are primarily responsible for hip flexion. The iliacus originates from the iliac fossa of the pelvis and inserts into the lesser trochanter of the femur. When the muscle contracts, it pulls the thigh upwards towards the body, achieving flexion at the hip joint.

Additionally, the iliacus plays a role in medial rotation due to its anatomical alignment and action points relative to the femur. When the thigh is flexed, as is the case during activities such as walking or climbing, the iliacus can contribute to moving the thigh inward, resulting in medial rotation.

The other muscles listed (psoas major, biceps brachii, and brachialis) do not fulfill this specific function. While the psoas major assists in hip flexion, it does not contribute as significantly to medial rotation as the iliacus. Meanwhile, the biceps brachii and brachialis are primarily involved in movements of the arm, not the thigh, making them unrelated to this function. Thus, the iliacus is correctly