**Core Concept**
Arthrography, stress radiography, and magnetic resonance imaging (MRI) are used to evaluate knee injuries, but the most appropriate investigation for a specific type of injury depends on the suspected pathology. In the case of a sports injury of the knee, the primary concern is often ligamentous or meniscal damage.

**Why the Correct Answer is Right**
Magnetic Resonance Imaging (MRI) is the investigation of choice for a sports injury of the knee because it provides detailed images of soft tissue structures, including ligaments, menisci, and cartilage. MRI is highly sensitive for detecting tears, contusions, and degenerative changes in these tissues. The high-resolution images allow for accurate assessment of the extent and location of the injury, which is crucial for diagnosing and managing sports-related knee injuries.

**Why Each Wrong Option is Incorrect**
**Option A:** Arthrography involves injecting a contrast agent into the joint space to visualize the internal structures of the knee. While it can be useful for detecting certain types of injuries, such as meniscal tears, it is not as sensitive as MRI for evaluating ligamentous or meniscal damage.

**Option B:** Stress radiography involves applying stress to the knee joint to evaluate the integrity of the ligaments. While it can be useful for detecting ligamentous instability, it is not as sensitive as MRI for detecting soft tissue injuries.

**Option D:** X-rays are useful for evaluating bone injuries, such as fractures or osteochondritis dissecans, but they are not sensitive for detecting soft tissue injuries, such as ligamentous or meniscal damage.

**Clinical Pearl / High-Yield Fact**
When evaluating a sports injury of the knee, it's essential to remember that the Lachman test is a reliable indicator of anterior cruciate ligament (ACL) injury, while the posterior drawer test is more specific for posterior cruciate ligament (PCL) injury.

**Correct Answer:** C. MRI.