This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Rheumatoid Ahritis, Subject name: Medicine
In rheumatoid ahritis the characteristic joint involvement is: 
Select and explain. 
A. Spine, B. Knee, C. Metacarpophalangeal joint, D. Hip joint
C: Metacarpophalangeal joint
Answer: C: Metacarpophalangeal joint 
Explanation: The hallmark feature of rheumatoid ahritis (RA) is persistent symmetric polyahritis (synovitis) that affects the hands and feet, although any joint lined by a synol membrane may be involved. In general, the small joints of the hands and feet are affected in a relatively symmetric distribution. In decreasing frequency, the metacarpophalangeal (MCP), wrist, proximal interphalangeal (PIP), knee, metatarsophalangeal (MTP), shoulder, ankle, cervical are involved.