<prompt> Metacarpophalangeal joints are most commonly affected in: March 2005OsteoahritisPsoriatic ahritisRheumatoid ahritisRheumatic fever  <response>: Ans. C: Rheumatoid Ahritis RA is characterized by diffuse cailage loss and erosion of bone and cailage. It stas in the synol membrane, with the initial processes of edema, neovascularization, and hyperplasia of the synol lining. Proliferation of synoviocytes and macrophages causes thickening of the synol lining and, together with lymphocytes, plasma cells, and mast cells, develops into pannus. Pannus is a sheet of invasive cellular tissue that is continuous with the synol lining. As a result of the higher propoion of synoviocytes and macrophages, pannus causes erosion of bone and cailage at the margin of joints. In the hands, the metacarpophalangeal MCP, proximal interphalangeal PIP, and thumb interphalangeal IP joints are most frequently involved. The distal interphalangeal DIP joints are involved only in the presence of a coexisting MCP or PIP disease. Tenosynovitis of the flexor tendons causes a reduction in finger flexion and grip strength. Nodular thickening in the tendon sheath may also produce a trigger finger. In osteoahritis and psoriatic ahritis, DIP is commonly involved. 