An 82 year old man develops a round, fluid-filled cystic structure on the back of his knee that limits the knee's mobility. This most likely represents an enlargement of which of the following structures?

The lesion is commonly called a Baker's cyst, and anatomically represents an enlarged semimembranosus bursa. Baker's cysts are more commonly seen at the extremes of age. The deep infrapatellar bursa is on the anterior aspect below the knee. The prepatellar bursa is anterior to the patella. The superficial infrapatellar bursa is on the anterior aspect of the leg below the knee. Ref: O'Keefe K.P., Sanson T.G. (2011). Chapter 278. Hip and Knee Pain. In J.E. Tintinalli, J.S. Stapczynski, D.M. Cline, O.J. Ma, R.K. Cydulka, G.D. Meckler (Eds), Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 7e.