Answer :USMLE step 2 MCQ 188:A 56 year old gardener complains of sudden pain with swelling behind the right knee

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Correct Answer: C

Explanation:
A popliteal or Baker's cyst a swelling behind the knee caused by escape of synovial fluid — can occur with septic arthritis, chronic inflammatory arthritis, osteoarthritis or arthritis associated with internal derangement. The prevalence of popliteal cysts ranges from 4.7% among orthopedic patient populations to 47.5% among patients with rheumatoid arthritis. The prevalence of cysts among patients with septic arthritis is unknown. Because of the correlation between popliteal cysts and knee-joint effusions, it is thought that cysts may develop as a result of a persistent knee effusion. A 1-way valve prevents popliteal cyst fluid from returning to the knee joint, and the increasing pressure may cause the cyst to rupture. When this occurs the associated pain and swelling in the calf mimics deep venous thrombosis, which is often wrongly diagnosed and treated. A crescentic hematoma below the medial or lateral malleoli of the ankle supports a diagnosis of a ruptured popliteal cyst; however, the best clue is sudden onset of calf pain.


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