Crystal Arthropathy (Gout or Pseudogout): This is a common condition in the elderly, characterized by sudden onset of joint pain and swelling, often without a clear precipitating injury. The symptoms can resolve on their own within 24 hours, which fits the patient's presentation.

Meniscal Tear or Ligament Sprain: Although the patient reported no injury, minor trauma can sometimes be forgotten or not recognized as significant. These conditions can cause knee pain and swelling.
Bursitis: Inflammation of the bursae around the knee can cause pain and swelling, and can occur without a specific injury.
Overuse or Degenerative Joint Disease: Osteoarthritis or overuse of the knee joint can lead to pain and swelling, especially in older adults.

Septic Arthritis: Although less likely, this is a medical emergency that requires prompt diagnosis and treatment. Bacterial infection of the joint can cause rapid onset of pain, swelling, and decreased range of motion.
Deep Vein Thrombosis (DVT): DVT can cause swelling and pain in the leg, and although it's more commonly associated with the calf, it should be considered, especially in older adults.
Osteonecrosis: Avascular necrosis of the bone can cause sudden onset of knee pain, especially if it involves the femoral condyle.

Hemarthrosis: Bleeding into the joint space, which could be due to a bleeding disorder, anticoagulant use, or a vascular malformation.
Neoplastic Processes: Tumors, either primary or metastatic, can cause joint pain and swelling, although this would be less common and might be accompanied by other systemic symptoms.
Inflammatory Arthritis (e.g., Rheumatoid Arthritis, Lupus): These conditions can cause joint pain and swelling but typically involve multiple joints and are accompanied by other systemic symptoms.