Osteoarthritis: Given the patient's age, osteoarthritis is a common cause of knee pain. It is characterized by degeneration of the joint cartilage and underlying bone, leading to pain and stiffness, especially after periods of rest or activity.

Meniscal Tear: A tear in the meniscus, which is the cartilage that cushions the joint, can cause knee pain, especially with twisting or bending movements.
Ligament Sprain: Sprains to the ligaments surrounding the knee, such as the anterior cruciate ligament (ACL) or medial collateral ligament (MCL), can cause pain and instability.
Tendinitis: Inflammation of the tendons, such as the patellar tendon, can cause knee pain, especially with activities that involve jumping or running.
Bursitis: Inflammation of the fluid-filled sacs (bursae) that cushion the knee joint can cause pain and swelling.

Septic Arthritis: A bacterial infection within the joint can cause severe pain, swelling, and redness, and requires prompt antibiotic treatment to prevent joint destruction.
Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the leg can cause knee pain and swelling, and can be life-threatening if it breaks loose and travels to the lungs.
Osteonecrosis: A condition where the bone tissue dies due to lack of blood supply, can cause knee pain and requires prompt treatment to prevent further damage.

Rheumatoid Arthritis: An autoimmune disease that causes inflammation and pain in the joints, including the knee.
Gout: A condition characterized by sudden, severe attacks of pain, swelling, and redness in the joint, often caused by elevated uric acid levels.
Pseudogout: A condition similar to gout, but caused by calcium pyrophosphate dihydrate crystals.
Osteochondritis Dissecans: A condition where a piece of cartilage and bone becomes detached from the joint surface, can cause knee pain and locking.