The correct answer is A - Complex regional pain syndrome - also known as Reflex Sympathetic Dystrophy (RSD), is a chronic pain condition that affects the limbs and is characterized by severe pain, swelling, and changes in skin color and temperature in the affected limb. The pain is typically out of proportion to the initial injury or trauma. The exact cause of CRPS is unknown, but it is thought to be related to an injury or trauma to the limb that triggers an abnormal response in the sympathetic nervous system. This response can cause an overproduction of pain signals, leading to chronic pain and other symptoms in the affected limb.

Scleroderma is a chronic autoimmune disease that affects the skin and connective tissues of the body. The disease is characterized by the thickening and hardening of the skin and damage to internal organs, blood vessels, and the immune system. The skin is not particularly sensitive to touch, and there are no sympathetic signs (e.g., sweating)

Venous obstruction may produce hand edema but usually causes more proximal swelling and does not tend to be so painful.

Rheumatoid arthritis (RA) is a chronic autoimmune disease that causes joint and surrounding tissues inflammation. The disease typically affects the small joints of the hands and feet but can also affect the knees, hips, and spine. There are also systemic manifestations. Again, the skin is not particularly sensitive to touch, and there are no sympathetic signs (e.g., sweating)