**Core Concept**
The diagnosis of a non-functioning kidney in a child requires a comprehensive assessment of renal function, anatomy, and potential underlying causes. This may involve imaging studies to evaluate kidney size, shape, and blood flow, as well as laboratory tests to assess renal function and detect potential complications.

**Why the Correct Answer is Right**
The best diagnostic test for a non-functioning kidney in a child is likely to be ultrasound with Doppler imaging. This modality provides detailed information about kidney size, shape, and blood flow, which is essential for diagnosing a non-functioning kidney. Ultrasound is also non-invasive, does not involve radiation, and is well-tolerated by children.

• Ultrasound with Doppler imaging can detect abnormalities in kidney size, shape, and blood flow, which are indicative of a non-functioning kidney.
• This modality can also assess the presence of any renal artery stenosis or thrombosis, which may be contributing to the non-functioning kidney.

**Why Each Wrong Option is Incorrect**
**Option A:** Renal function tests (RFTs) may provide some information about renal function, but they are not specific enough to diagnose a non-functioning kidney.
• RFTs, such as serum creatinine and urea levels, may be abnormal in a child with a non-functioning kidney, but they do not provide information about kidney anatomy or blood flow.

**Option B:** Computed Tomography (CT) scans may be useful in some cases, but they involve radiation and are not the first-line diagnostic test for a non-functioning kidney.
• CT scans can provide detailed information about kidney anatomy and blood flow, but they are not as sensitive as ultrasound for detecting abnormalities in children.

**Option C:** Magnetic Resonance Imaging (MRI) may be useful in some cases, but it is not the best initial diagnostic test for a non-functioning kidney.
• MRI can provide detailed information about kidney anatomy and blood flow, but it is more expensive and less widely available than ultrasound.

**Clinical Pearl / High-Yield Fact**
In children, a non-functioning kidney is often due to congenital anomalies or acquired conditions, such as urinary tract infections or renal artery stenosis. A thorough evaluation of renal function, anatomy, and potential underlying causes is essential for making an accurate diagnosis.

**Correct Answer:** C. Ultrasound with Doppler imaging is the best diagnostic test for a non-functioning kidney in a child.