1 / 10 RPI Deficiency: Ribose-5-Phosphate Isomerase (RPI) deficiency is considered one of the rarest diseases in the world. (Image: Cell Press)2 / 10 Field’s Disease: Field’s disease is an extremely rare neuromuscular disorder, with only two known cases—twin sisters from Wales. (Wikimedia Commons)ADVERTISEMENT5 / 10 Aquagenic Urticaria (Water Allergy): Aquagenic urticaria is an extremely rare condition where contact with water causes red, itchy hives. (File Photo)ADVERTISEMENT7 / 10 Lesch–Nyhan Syndrome: Lesch–Nyhan Syndrome is a rare genetic disorder caused by excess production of uric acid. (File Photo)ADVERTISEMENT9 / 10 Harlequin Ichthyosis: Harlequin Ichthyosis is a severe genetic skin disorder where the skin forms thick, plate-like scales.