What is the likely diagnosis for a patient with left lateral knee pain, tenderness over the medial joint line, and a locking sensation?

The presentation of left lateral knee pain, tenderness over the medial joint line, and a locking sensation is highly suggestive of a torn meniscus. In particular, the location of tenderness over the medial joint line aligns with common symptoms associated with medial meniscus tears, which often result from twisting injuries to the knee.

The locking sensation occurs when a fragment of the torn meniscus becomes displaced and obstructs the normal range of motion in the knee joint. This specific symptom is characteristic of meniscal tears, as the torn tissue can catch in the knee joint during movement, leading to the sensation that the knee is "locked".

In contrast, patellar tendinitis typically presents as anterior knee pain and does not usually cause joint line tenderness or locking sensations. A ligament sprain may lead to pain and swelling but would not specifically result in medial joint line tenderness or the locking symptom linked to meniscal injuries. Joint effusion refers to swelling due to excess fluid in the knee joint; while it can occur with various knee injuries, it is not associated with the specific locking sensation observed here.