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This is most likely a ligament injury which is giving you such pain. At the age of 76 these supporting structure of the knee joints can get weak and get damaged easily by such minor injury. Meniscal injury is another possibility although the weight bearing is usually compromised. The most likely problem here is an ACL tear. You should take some anti inflammatory like Ibuprofen with Omeprazole. Avoid excessive weight bearing. Use ice and knee support .You will need to see the doctor if the problem does not get better in a few days time
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