Osteoarthritis (OA): Given the patient's history of knee pain, fluid buildup, and cortisone shot, OA is a likely culprit. The absence of reported trauma and the chronic nature of the symptoms also support this diagnosis. OA is a common cause of knee pain and swelling, especially in individuals who engage in activities like golfing.

Meniscal Tear or Degeneration: The patient's age and history of knee pain, along with the use of a knee brace during golfing, suggest possible meniscal issues. Although no trauma is reported, meniscal tears can occur without significant injury, especially in the context of degenerative changes.
Ligamentous Sprain or Strain: Despite no reported trauma, minor injuries can occur during activities like golfing, leading to ligament sprains or strains. These could contribute to knee swelling and discomfort.
Bursitis: The patient's history of fluid buildup and cortisone shot could also suggest bursitis, especially if the bursae around the knee are inflamed.

Septic Arthritis: Although less likely without a clear history of trauma or infection, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. The presence of significant knee swelling and discomfort warrants consideration of this diagnosis.
Deep Vein Thrombosis (DVT): DVT can cause leg swelling and discomfort and is a potentially life-threatening condition if not recognized and treated promptly.
Tumor (Benign or Malignant): While rare, tumors around the knee can cause swelling and pain. A thorough examination and possibly imaging studies are necessary to rule out this possibility.

Rheumatoid Arthritis (RA) or Other Autoimmune Arthropathies: These conditions can cause knee swelling and pain but are less common than OA or other degenerative conditions. A systemic review of symptoms and possibly serologic tests would be needed to consider these diagnoses.
Hemarthrosis: Spontaneous bleeding into the knee joint can occur, especially in patients with bleeding disorders or those on anticoagulant therapy. This would be an unusual presentation without a known underlying condition or trauma.
Pigmented Villonodular Synovitis (PVNS): A rare condition characterized by swelling and pain due to synovial proliferation. It would be considered if other more common diagnoses are ruled out and if there are characteristic findings on imaging studies.