**Core Concept**
The acutely injured knee of a 27-year-old athlete requires a reliable diagnostic test to assess the extent of injury, particularly to the menisci, ligaments, and articular cartilage. Magnetic Resonance Imaging (MRI) is a valuable tool in evaluating the internal structures of the knee joint, providing detailed images of soft tissues and bone.

**Why the Correct Answer is Right**
MRI is the most reliable test for an acutely injured knee because it can detect a wide range of injuries, including meniscal tears, ligament sprains or tears, and bone bruises. MRI uses a strong magnetic field and radio waves to produce detailed images of the internal structures of the knee, allowing for accurate diagnosis and planning of treatment. This is particularly useful in the acute setting, where a thorough understanding of the injury is crucial for guiding management and preventing further damage.

**Why Each Wrong Option is Incorrect**
**Option A:** X-ray - While X-rays can detect bone fractures and some types of ligament injuries, they are not reliable for detecting soft tissue injuries such as meniscal tears or ligament sprains, making them less useful in the acute setting.
**Option B:** Arthroscopy - Arthroscopy is a valuable tool for diagnosing and treating knee injuries, but it is an invasive procedure that requires surgical intervention. It is typically reserved for cases where conservative management has failed or when surgical intervention is necessary.
**Option C:** Physical examination - While a thorough physical examination is essential in evaluating knee injuries, it may not be sufficient to detect all types of injuries, particularly those that are not apparent on initial examination.

**Clinical Pearl / High-Yield Fact**
In the acute setting, it's essential to remember that a normal X-ray does not rule out a meniscal tear or ligament injury. A thorough physical examination and MRI are often necessary to accurately diagnose and manage knee injuries.

**Correct Answer:** C. Arthroscopy.