Osteoarthritis (OA): The patient's age, symptoms worsening with activity such as bending or climbing stairs, and tenderness over the medial knee joint are classic for OA. The condition is very common in this age group and typically affects weight-bearing joints.

Meniscal Tear: Given the patient's age and the nature of the pain, a meniscal tear is possible, especially if the pain is localized to the medial aspect of the knee. Activities that involve twisting or bending can exacerbate symptoms.
Ligament Sprain: Although the patient denied any falls, a ligament sprain could still be considered, especially if there was a minor incident or overuse that didn't seem significant enough to report.
Bursitis: Inflammation of the bursae around the knee can cause pain and tenderness, especially with activities like climbing stairs. The medial aspect of the knee has several bursae that could be involved.

Septic Arthritis: Although less likely without a history of fever, recent travel, or immune compromise, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. The presence of significant pain, especially with movement, and tenderness over the joint should raise suspicion.
Deep Vein Thrombosis (DVT): While DVT typically presents with swelling and warmth of the leg, it can sometimes cause pain around the knee, especially if the clot is located in the proximal veins. Given the patient's age, this is a critical diagnosis not to miss due to its potential for severe complications.
Osteonecrosis: Also known as avascular necrosis, this condition can cause knee pain and is more common in older adults. It might not be the first consideration but is important due to its potential for significant morbidity if not addressed.

Tumor (Primary or Metastatic): While rare, tumors can cause localized pain and should be considered, especially in older adults who may have a higher risk of cancer.
Inflammatory Arthritis (e.g., Rheumatoid Arthritis, Psoriatic Arthritis): These conditions can cause knee pain and are typically associated with other systemic symptoms or findings. However, they are less common in new onset cases at this age without prior history.
Complex Regional Pain Syndrome (CRPS): This is a rare condition characterized by intense burning or aching pain in the hands or feet after an injury or trauma. It could be considered if the pain is out of proportion to the expected injury or if there are other characteristic symptoms like changes in skin temperature or color.