Which condition is characterized by the lower and upper extremities in a position of spastic extension, adduction, and internal rotation?

The condition characterized by lower and upper extremities in a position of spastic extension, adduction, and internal rotation is decerebrate rigidity. This state typically arises from severe damage to the brainstem, particularly in areas that control muscle tone and reflexes. In decerebrate rigidity, muscles exhibit an abnormal posture where the arms are extended and the legs are too, which reflects increased extensor tone and decreased flexor tone. This pattern demonstrates the body's response to neurological impairment and results from the loss of higher motor control, leaving extensors via the brainstem to dominate.

In contrast, decorticate rigidity results in flexion of the arms and extension of the legs, demonstrating a different neurological impact. Athetosis involves a continuous movement pattern characterized by twisting and uncontrolled movements, which does not include the rigidity described. Conversion disorder is a psychological condition presenting physical symptoms without a neurological basis, making it unrelated to the spastic posture typical of decerebrate rigidity. Thus, the correct answer identifies the specific and characteristic muscular alignment seen in decerebrate rigidity, distinguishing it from other neurological conditions.