Final answer:
A medial collateral ligament (MCL) injury is caused by a forceful blow to the lateral side of the extended knee, which can stretch or damage the tibial collateral ligament and the medial meniscus. In severe cases, the anterior cruciate ligament (ACL) may also be ruptured.
Step-by-step explanation:
A medial collateral ligament (MCL) injury is caused by a forceful blow to the lateral side of the extended knee. This blow causes the medial side of the joint to open, resulting in stretching or damage to the tibial collateral ligament. In more severe cases, a stronger blow can tear the ligament and also damage the medial meniscus. The anterior cruciate ligament (ACL) may also be ruptured, resulting in what is known as a 'terrible triad' injury.