A very low vitamin D level (6.5 ng/mL) can cause bone pain, muscle weakness, and joint discomfort. In your father's case, the swelling and pain in the knee and ankle may be due to a combination of severe Vitamin D Deficiency and age-related joint wear (degenerative changes), which is often related to Osteoarthritis. Treatment usually includes vitamin D supplementation (often high-dose vitamin D for a few weeks, followed by maintenance doses), calcium if needed, pain-relief medicines, and gentle physiotherapy or exercises to strengthen the joints. Sunlight exposure and a balanced diet also help improve vitamin D levels. If swelling persists, it is best to follow up with the doctor to rule out other causes and adjust treatment accordingly.