Which muscle is primarily responsible for flexing and externally rotating the hip?

The iliopsoas muscle is primarily responsible for flexing the hip joint and plays a crucial role in external rotation of the hip. This muscle group comprises the psoas major and the iliacus muscles, which work together to bring the thigh towards the abdomen, effectively facilitating hip flexion. Additionally, the iliopsoas is instrumental in external rotation due to its anatomical positioning and action on the femur during specific movements, such as when lifting the thigh or executing movements that require rotation outward.

While other muscles mentioned may assist with movement around the hip, their primary functions differ. For instance, the gluteus maximus is primarily involved in extending and internally rotating the hip rather than flexing it. The hamstrings are mainly responsible for hip extension and knee flexion, while the rectus femoris, which is part of the quadriceps group, primarily functions to extend the knee and also flex the hip, but it is not as effective in external rotation as the iliopsoas. Therefore, the iliopsoas is the correct choice as it directly addresses both the flexion and the external rotation of the hip.