Madelung's deformity is that of excessive ulnar/palmar angulation of the distal radius caused by impaired growth of the volar and ulnar aspect of the distal radial physis. It may be caused by either a bony lesion in the palmar/ulnar corner of the distal radial physis or an abnormal radial-carpal ligament (Vicker's ligament). The other answers do not cause Madelung's deformity. Leri-Weill dyschondrosteosis is a rare genetic disorder caused by mutation in the SHOX gene that causes mesomelic dwarfism with associated Madelung's defomity of the forearm.
Illustration A is a radiographic example of Madelung's deformity.
Waters PM. The Upper Limb. In Morrissy RT, Weinstein SL (EDS): Lovell and Winters Pediatric Orthopaedics ED, 6. Philadelphia, PA, Lipincott Williams & Wilkins, 2006. PP 921-986
James MA, Bednar M. Deformities of the Wrist and Forearm. In Green DP, Hotchkiss, RN, Pederson, WC (EDS): Greens Operative Hand Surgery, ED 5. New York, NY. Elsevier Churchill Livingstone, 2005. PP 1469-1505