**Core Concept**
The clinical presentation of a painful limp in a 6-year-old boy, with tenderness in the femoral triangle and limited hip movements, suggests Perthes disease or Transient Synovitis as possible causes. However, the normal X-ray makes it difficult to distinguish between these two conditions, necessitating further investigation.

**Why the Correct Answer is Right**
The next investigation should be a hip ultrasound to evaluate for joint effusion, which is a characteristic feature of Transient Synovitis. Transient Synovitis is an inflammatory condition of the hip joint, often seen in children, characterized by joint pain, swelling, and limited mobility. The presence of joint effusion on ultrasound can help differentiate Transient Synovitis from Perthes disease, which typically shows no joint effusion.

**Why Each Wrong Option is Incorrect**
**Option A:** Arthrocentesis would be a more invasive procedure and is not the first line of investigation for a child with a painful limp.
**Option B:** CT scan or MRI might be considered in cases where the diagnosis is still unclear after ultrasound, but they are not the next best step.
**Option C:** X-ray is already normal, and repeating it would not provide any new information.

**Clinical Pearl / High-Yield Fact**
In children presenting with hip pain, it's essential to consider the possibility of Transient Synovitis, which can mimic Perthes disease clinically but is often associated with joint effusion on ultrasound.

**Correct Answer:** B. MRI.