Rotation in a flexed position only occurs at which of the following joints?

The spine is the correct answer because it is the only joint among the options provided that allows for rotation while in a flexed position. When the spine is flexed, its vertebrae can rotate around the central axis, enabling movements such as twisting or turning the torso.

In contrast, while the hip joint does offer some range of motion, its primary movements occur in extension and flexion, and it does not typically undergo significant rotation when fully flexed due to structural constraints. The elbow joint, while capable of movement, primarily functions as a hinge joint, allowing for flexion and extension but very little rotational movement in a flexed position. The knee joint, similarly, is mainly designed for flexion and extension and does not facilitate rotation when in a flexed state, as its anatomic structure limits this capability. Thus, the spine stands out as the joint that effectively allows for rotational movement when in a flexed position.