A 18 yrs old boy was playing football, when he suddenly twisted his knee on the ankle and he fell down. He got up after 10 minutes and again staed playing, but next day his knee was swollen and he could not move it. The most probable cause is:

Injury to the medial meniscus is the most frequent internal derangement of the knee joint. Clinical findings include swelling, pain, and varying degrees of restriction of flexion or extension. True locking (inability to fully extend the knee) is highly suggestive of meniscal tear. A marginal tear permits displacement of the medial fragment into the intercondylar region (bucket-handle tear) and prevents either complete extension or complete flexion. Ref: Srinivasan R.C., Tolhurst S., Vanderhave K.L. (2010). Chapter 40. Ohopedic Surgery. In G.M. Dohey (Ed), CURRENT Diagnosis & Treatment: Surgery, 13e.