MRCS-1279

A 26-year-old man is playing football when he slips during a tackle. His left knee becomes painful immediately after. Several hours later, he notices that the knee has become swollen. Following a course of NSAIDs and rest, the situation improves. However, he presents to the clinic with recurrent pain. On examination, it is impossible to fully extend the knee, although the patient is able to do so when asked. What is the most likely injury?