Which condition is the most likely cause of pain and stiffness in a 64-year-old woman with joint space narrowing and subchondral cyst formation?

The condition that best explains pain and stiffness in a 64-year-old woman characterized by joint space narrowing and subchondral cyst formation is osteoarthritis. This degenerative joint disease is particularly prevalent in older adults and is associated with wear and tear of the cartilage, leading to a decrease in joint space.

In osteoarthritis, the loss of cartilage results in the bone underneath becoming exposed and potentially forming cysts in response to the altered joint mechanics and increased stress. These subchondral cysts can be visualized on imaging studies, which helps differentiate osteoarthritis from other arthritic conditions.

Additionally, the symptoms of pain and stiffness, especially after periods of inactivity (often referred to as "morning stiffness"), are characteristic of osteoarthritis. As the disease progresses, the stiffness typically improves with movement, which further distinguishes it from inflammatory arthritis.

While the other conditions listed can also cause joint pain and stiffness, they usually present with different clinical features. For example, acute gouty arthritis often presents with sudden onset of severe pain and swelling in a single joint, rheumatoid arthritis typically affects multiple joints symmetrically with systemic symptoms, and septic arthritis often shows evidence of infection with acute onset and is usually accompanied by significant redness and swelling.