**Core Concept**
The knee joint is a complex structure composed of bones, ligaments, and cartilage. Cartilage injuries in the knee can be challenging to diagnose, and a thorough investigation is necessary to determine the extent of the damage.

**Why the Correct Answer is Right**
The initial investigation for knee cartilage injury typically involves a detailed clinical history, physical examination, and imaging studies. A thorough clinical examination may include assessment of knee stability, range of motion, and joint effusion. Imaging studies such as X-rays, magnetic resonance imaging (MRI), and arthroscopy are essential in confirming the diagnosis and evaluating the extent of cartilage damage. MRI is particularly useful in detecting cartilage lesions, as it can provide detailed images of the cartilage and surrounding bone.

**Why Each Wrong Option is Incorrect**
**Option A:** X-ray is not the primary imaging modality for cartilage injuries, as cartilage does not show up well on X-rays.
**Option B:** Arthroscopy is a diagnostic and therapeutic procedure that can be used to visualize the joint and repair cartilage damage, but it is not the first line of investigation.
**Option C:** Physical examination is an essential part of the evaluation, but it is not a specific diagnostic test for cartilage injuries.

**Clinical Pearl / High-Yield Fact**
When evaluating knee cartilage injuries, it's essential to remember that early detection and treatment can significantly improve outcomes. A thorough history, physical examination, and imaging studies are crucial in confirming the diagnosis and guiding treatment.

**Correct Answer: D. MRI is the best initial imaging modality for knee cartilage injuries, as it provides detailed images of the cartilage and surrounding bone.**