A 22-year-old woman with six months of left knee pain after a basketball game a year ago, with giving way and worse on stairs, patellar tenderness, BMI 46. Which diagnosis is most likely?

• A. Prepatellar bursitis
• B. Chondromalacia patella
• C. ACL tear
• D. Medial meniscus tear

Answer: (C)

The main idea here is that persistent knee instability after a twisting injury points to an ACL tear. When the ACL is torn, the tibia can translate forward relative to the femur, especially during weight-bearing activities or movements that involve deceleration or change of direction. This creates a sensation of the knee giving way or buckling, which is most noticeable when climbing or descending stairs and during sports. The patellar tenderness noted can occur with various knee issues, but it doesn’t explain the instability that has persisted since the injury.

Prepatellar bursitis would present with anterior knee swelling and tenderness over the kneecap, typically from kneeling, not with recurrent giving way. Patellofemoral pain (chondromalacia) causes anterior knee pain and pain with stairs or squatting and patellar tenderness, but instability or giving way is not a hallmark feature. Medial meniscus tears cause joint-line tenderness and mechanical symptoms like locking or catching rather than a primary sense of instability. An ACL tear best fits the symptom of giving way, especially in the setting of a basketball pivoting/in twisting injury and ongoing instability during stairs.