**Core Concept**
The patient presents with Acute Renal Failure (ARF) characterized by complete anuria, indicating a severe reduction in urine output, but with a normal ultrasound, which rules out obstructive causes such as kidney stones or tumors. This scenario suggests a non-obstructive cause of renal failure.

**Why the Correct Answer is Right**
Given the clinical presentation and normal ultrasound, the next step is to differentiate between prerenal and intrinsic renal causes of ARF. Since the question doesn't specify the correct answer choice, a typical next investigation would involve assessing renal function and structure more closely, possibly through imaging that can evaluate renal perfusion or function directly.

**Why Each Wrong Option is Incorrect**
**Option A:** Without knowing the specific choice, we can't directly address why it's incorrect, but typically, options that suggest immediate intervention without diagnosis might be premature.
**Option B:** Similarly, without specifics, but options that don't contribute to diagnosing the cause of anuria might not be the best next step.
**Option C:** Again, lacking specifics, but if this option involves advanced or invasive testing without initial evaluation, it might not be appropriate.
**Option D:** This option, like the others, would depend on what it entails, but if it's not directed towards diagnosing the cause of renal failure, it would be less ideal.

**Clinical Pearl / High-Yield Fact**
In patients with acute renal failure and anuria, it's crucial to quickly distinguish between prerenal, intrinsic renal, and postrenal causes. A normal ultrasound helps to rule out postrenal causes, making further investigations aimed at prerenal or intrinsic causes critical.

**Correct Answer:** Unfortunately, without the specific answer choices provided, a precise answer cannot be given. However, typically, the next best step after ruling out obstruction would involve assessing renal function or perfusion directly.