A patient presented with ARF with complete anuria, but a normal ultrasound. Next investigation is:
**Core Concept**
In a patient with acute renal failure (ARF) and complete anuria, but a normal ultrasound, the next investigation is crucial to identify the underlying cause. This scenario suggests that the kidneys may be structurally normal, but the renal function is impaired due to other reasons such as obstructive uropathy, vasopressin-sensitive diabetes insipidus, or post-renal causes.
**Why the Correct Answer is Right**
The correct investigation in this scenario would be a **renal function test**, specifically a **renal scintigraphy** or **diuretic renography**. These tests assess the function and drainage of the kidneys, helping to identify potential obstructions or functional abnormalities. The normal ultrasound rules out structural abnormalities, making functional assessment the next step.
**Why Each Wrong Option is Incorrect**
* **Option A:** A CT scan or MRI might be useful in identifying structural abnormalities, but they are not the next best investigation in this scenario, given the normal ultrasound.
* **Option B:** A urine culture is essential in patients with a history of urinary tract infections, but it does not directly address the issue of complete anuria and normal ultrasound findings.
* **Option C:** A renal biopsy is an invasive procedure that would be considered if structural abnormalities or specific renal diseases are suspected, but it is not the next best investigation in this scenario.
**Clinical Pearl / High-Yield Fact**
In patients with ARF and complete anuria, but a normal ultrasound, always consider vasopressin-sensitive diabetes insipidus as a potential cause, which can be diagnosed with a water deprivation test.
**Correct Answer:** C.