Which of the following joint is characteristically involved in RA. -
Rheumatoid arthritis (RA)                                                     

RA is a chronic multisystem disorder that may affect many tissues and organs but principally attacks the joint, producing a nonsuppurative proliferative and inflammatory synovitis that often progresses to destruction of the articular cartilage and ankylosis of the joint.
Symptoms usually develop in the small joints of the hands, (i.e. metacarpophalangeal & proximal interphalangeal joint), and feet (metatarsophalangeal & interphalangeal) joint.

Articular manifestations in rheumatoid arthritis.

Rheumatoid arthritis can affect any of the synovial joints.
Most commonly the disease starts classically in the joints of hand and feet i.e. PIP, MCP and MTP with the involvement of large joints occurring later.

A. The hands

These are the major site of involvement in almost all patients with rheumatoid arthritis.
Of particular importance is the sparing of the distal interphalangeal joint.
Most commonly the disease starts in the proximal interphalangeal joint. (PIP), and metacarpophalangeal joint (MCP).

Deformities seen with hand involvement

Swan Neck deformity  → hyperextension of PIP joint.
Buttonhole deformity → hyperextension of MCP joint.

B. The Feet

Metatarsophalangeal joints are particularly involved in almost all cases of rheumatoid arthritis and are second only to hand involvement.

C. Wrist

Involved in most patients; radial deviation is the rule.

D. Large Joints

Involvement of large joint (knee, elbows, hips and shoulders is common but occurs somewhat later than small joint).

Symmetric involvement of the joint is also an important feature of articular involvement in rheumatoid arthritis. The radiological hallmark of rheumatoid arthritis is:

Juxtaarticular osteopenia
Bone erosions
Narrowing of joint space from loss of articular cartilage (this is typical of any inflammatory arthritis).
Spine
Sacroiliac joint
Metacarpophalangeal joint and proximal interphalangeal joint
Wrist joint