**Core Concept**
The investigation of choice for a sport injury of the knee involves a comprehensive evaluation of the joint's anatomy, function, and potential damage. This includes assessing the integrity of ligaments, tendons, and menisci, as well as identifying any potential fractures or dislocations.

**Why the Correct Answer is Right**
Magnetic Resonance Imaging (MRI) is the preferred modality for evaluating knee injuries in athletes. It provides detailed images of soft tissues, including ligaments, tendons, and menisci, which are often injured in sport-related trauma. MRI can identify tears, sprains, and other types of damage, allowing for accurate diagnosis and targeted treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** X-ray is not the investigation of choice for knee injuries because it primarily evaluates bone structures and may not detect soft tissue damage or ligament sprains.
**Option B:** CT scan is not the preferred modality for knee injuries, as it provides less detail of soft tissues compared to MRI and is more suited for evaluating bone fractures.
**Option C:** Ultrasound is limited in its ability to evaluate the complex anatomy of the knee joint and is not as sensitive as MRI for detecting soft tissue injuries.

**Clinical Pearl / High-Yield Fact**
When evaluating a sport-related knee injury, it's essential to remember that MRI is the gold standard for diagnosing soft tissue damage, including ligament sprains and meniscal tears. This is particularly important in athletes, where accurate diagnosis and prompt treatment can significantly impact recovery and return to play.

**Correct Answer: C. MRI**