**Core Concept**
The clinical presentation of a painful limp in a child, particularly with tenderness in the femoral triangle and limited hip movements, raises suspicion of Perthes disease, a condition characterized by avascular necrosis of the femoral head. This condition primarily affects children between 4 to 10 years of age.

**Why the Correct Answer is Right**
In the case of Perthes disease, the femoral head becomes ischemic due to interruption of the blood supply, usually from the medial circumflex femoral artery. The clinical examination findings, such as tenderness in the femoral triangle and limited hip movements, are consistent with this condition. A normal X-ray at this stage does not rule out Perthes disease, as the early changes may not be visible on plain radiographs. Further diagnostic workup, such as an MRI or bone scan, is required to confirm the diagnosis. Since the X-ray is normal, the next step is to proceed with further imaging to assess for early signs of Perthes disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Ordering a CT scan would not be the next step, as it exposes the child to radiation and does not provide additional information over MRI or bone scan in the context of suspected Perthes disease.

**Option B:** Proceeding directly to surgical intervention without further diagnostic workup is not recommended, as it may not be necessary and could lead to unnecessary risks for the child.

**Option C:** Discharging the child without further investigation would be inappropriate, given the persistent symptoms and the need to rule out potential underlying conditions that may require treatment.

**Option D:** Ordering a blood test to check for infection or other systemic conditions may not be directly relevant to the clinical presentation and would not address the primary concern of Perthes disease.

**Clinical Pearl / High-Yield Fact**
In cases of suspected Perthes disease, it's essential to remember that the initial X-ray may appear normal, and further imaging is necessary to confirm the diagnosis. A delay in diagnosis can lead to poor outcomes, emphasizing the need for prompt and accurate diagnosis.

**Correct Answer: D. Discharging the child without further investigation would be inappropriate, given the persistent symptoms and the need to rule out potential underlying conditions that may require treatment.**