The answer to the Medical Challenge this week is #3 - Graves' disease.
The skin changes seen on the lower legs are typical of Graves' dermopathy. The skin has a leathery texture, thickening and fissuring.
Graves' disease, named after the Irish doctor Robert James Graves, who first described it in 1835 is caused by an overactive thyroid gland. Thyroid auto-antibodies stimulate thyroid hormone