**Core Concept**
The patient presents with acute renal failure, and an ultrasound of the abdomen is normal, indicating no obvious obstructive cause. The next step involves investigating the renal function and structure in more detail. **Acute kidney injury (AKI)** can result from various causes, including pre-renal, intrinsic renal, and post-renal causes.

**Why the Correct Answer is Right**
Given the normal ultrasound, the focus shifts to assessing the renal parenchyma and function. A **CT scan** or **renal biopsy** might be considered, but these are more invasive or not initially necessary. The most appropriate next step, considering the options provided (though not explicitly listed), would typically involve assessing renal function and structure through imaging that can provide more detail than ultrasound, such as **Doppler ultrasound** to assess renal blood flow or a **CT scan** without contrast to evaluate the renal parenchyma, but without the options, the best guess for a next useful investigation after a normal ultrasound would be to look for signs of renal damage or disease through a **renal biopsy** or a detailed **imaging study**.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option provided, it's challenging to directly address why it might be incorrect, but if it involved an unnecessary or invasive procedure without clear indication, it would not be the next best step.
**Option B:** Similarly, without specifics, if this option involved a test that does not directly contribute to understanding the cause of acute renal failure after a normal ultrasound, it would be less useful.
**Option C:** If this option suggested a procedure or test that is not typically used in the initial evaluation of acute renal failure, it would be incorrect.
**Option D:** If this option recommended a test that is not directly relevant to assessing renal function or structure in the context of acute renal failure, it would not be the best choice.

**Clinical Pearl / High-Yield Fact**
In patients with acute renal failure and a normal ultrasound, considering the underlying cause (pre-renal, intrinsic renal, or post-renal) is crucial. A key point to remember is that **intrinsic renal causes** often require more detailed evaluation, including potential biopsy, to determine the exact nature of the renal injury.

**Correct Answer:** Correct Answer: D. CT scan