Sagittal MRI Knee — Ligaments and Menisci

A 22-year-old female basketball player sustains a non-contact injury while cutting sharply during a game. She reports hearing a loud "POP" at the moment of injury, followed by immediate swelling of the knee and a sensation that "the knee gave way." On examination, the Lachman test (performed at 30° knee flexion) demonstrates significant anterior tibial translation. MRI of the knee in the sagittal plane shows a complete tear of the structure marked **A**. Which of the following best describes the primary biomechanical function that is now compromised in this patient?