Osteoarthritis (OA): Given the patient's age and the presence of mild degenerative changes in the medial compartment on X-ray, OA is a likely cause of her knee pain. The small knee effusion can also be associated with OA.

Meniscal Tear: The presence of a small knee effusion and degenerative changes could also suggest a meniscal tear, especially if the patient has a history of trauma or twisting injury.
Ligamentous Sprain: A sprain of the medial collateral ligament (MCL) or other ligaments around the knee could cause pain and effusion, and might not be visible on X-ray.
Patellofemoral Pain Syndrome: This condition, characterized by pain around or behind the patella, is common in women and could be a cause of her knee pain, especially if she has recently increased her activity level.

Septic Arthritis: Although the X-ray does not show signs of infection, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. Risk factors such as recent travel, immune compromise, or intravenous drug use would increase the suspicion.
Osteonecrosis: While less common, osteonecrosis (avascular necrosis) of the femoral condyle could present with knee pain and might not be evident on initial X-rays, requiring further imaging like MRI for diagnosis.
Tumor: Although the X-ray does not show a focal osseous lesion, it's crucial not to miss a bone tumor, which could cause pain and effusion. Further imaging might be necessary if the pain persists or worsens.

Pigmented Villonodular Synovitis (PVNS): A rare condition characterized by synovial proliferation and pigmentation, PVNS could cause chronic knee pain and effusion.
Hemophilic Arthropathy: In patients with a history of hemophilia, recurrent hemarthrosis can lead to degenerative changes and chronic pain.
Neuropathic Pain: Rarely, knee pain can be referred from a neuropathic source, such as a lumbar radiculopathy, and would require a thorough neurological examination and possibly further diagnostic tests.