A 62-year-old woman comes to the emergency department due to acute leg pain. The patient, who recently started an exercise program for weight loss, felt pain at the right knee and posterior calf while walking on a treadmill. She subsequently developed swelling at the calf and right ankle. Medical history is notable for type 2 diabetes mellitus, hypertension, and hypercholesterolemia. The patient does not use tobacco or alcohol. Temperature is 36.5 C (97.7 F), blood pressure is 166/88 mm Hg, and pulse is 90/min. BMI is 41 kg/m². Examination shows tenderness and induration at the medial head of the gastrocnemius. There is moderate pitting edema at the ankle and a crescent-shaped patch of ecchymosis at the medial malleolus. Which of the following is the most likely cause of this patient's symptoms?

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11ec25c8_d963_b5d1_bc81_c7e8f627832f_MD0003_00 This patient likely has a ruptured popliteal (Baker) cyst. A popliteal cyst is due to extrusion of synovial fluid from the knee joint into the gastrocnemius or semimembranosus bursa through a communication between the joint and the bursa. Excessive synovial fluid formation (eg, due to osteoarthritis or rheumatoid arthritis) and positive pressure in the knee during extension can cause passage of fluid into the bursa and gradual enlargement of the cyst. Popliteal cysts are often asymptomatic and present as a chronic, painless bulge behind the knee. The diagnosis is usually apparent on examination, with a soft mass in the medial popliteal space that is most noticeable with knee extension and less prominent with flexion. Rupture of a popliteal cyst (eg, following strenuous exercise) can cause posterior knee and calf pain, with tenderness and swelling of the calf resembling deep venous thrombosis (DVT). An arc of ecchymosis is often visible distal to the medial malleolus ("crescent sign"). Ultrasound can rule out DVT and confirm the popliteal cyst. (Choice A) Pes anserinus pain syndrome (anserine bursitis) presents with subacute medial knee pain. Examination shows a well-defined area of tenderness over the medial tibial plateau below the joint line. (Choice C) Stress fractures of the tibia present with progressive pain at the shin and focal tenderness over the fracture site. (Choice D) Tears of the medial collateral ligament are caused acutely by a twisting injury or blow to the lateral knee. Examination findings include tenderness at the medial joint line and valgus laxity. (Choice E) Tears of the medial meniscus result from a twisting force on a fixed foot. Patients report a popping sound followed by acute pain. Typical examination findings include a small effusion and crepitus, locking, or catching with range of motion. (Choice F) The onset of symptoms with exercise, posteromedial tenderness, and ecchymosis at the medial malleolus make ruptured popliteal cyst more likely than DVT. Educational objective: A popliteal (Baker) cyst is due to extrusion of synovial fluid from the knee joint into the gastrocnemius or semimembranosus bursa, and is most common in patients with underlying arthritis. Popliteal cysts may present as a painless bulge in the popliteal space, but cyst rupture can cause acute pain in the calf. References: Popliteal cysts: a current review. (http://www.ncbi.nlm.nih.gov/pubmed/25102502)

A 62-Year-Old Woman Comes to the Emergency Department Due to Acute