**Question:** A patient presents with acute renal failure with normal ultrasound abdomen report. Next most useful investigation is:

A. Serum creatinine levels
B. Serum electrolytes levels
C. Renal artery Doppler study
D. Serum urea levels

**Core Concept:** Acute renal failure is a rapid onset decrease in kidney function leading to impaired filtration and excretion of waste products. The initial diagnostic steps involve clinical evaluation, laboratory tests, and imaging studies.

**Why the Correct Answer is Right:** Serum creatinine levels (D) are essential in evaluating kidney function as they help determine the glomerular filtration rate (GFR), quantifying the degree of kidney dysfunction. Normal renal function is indicated by a serum creatinine level below 1.1 mg/dL in males and below 0.9 mg/dL in females.

**Why Each Wrong Option is Incorrect:**

A. Serum creatinine levels are critical, but other laboratory tests like serum electrolytes levels (B) and serum urea levels (D) are also essential in assessing renal function. However, these tests alone cannot confirm the cause of acute renal failure.

C. A renal artery Doppler study (C) is typically performed to evaluate renal artery blood flow and stenosis in cases of ischaemic renal disease. It is not the next most helpful investigation in a patient with normal abdominal ultrasound findings.

**Clinical Pearl:** Regular monitoring of serum creatinine levels, electrolytes, and urea levels is crucial in the assessment of renal function and the identification of potential causes of acute renal failure, such as hypovolemia, nephrotoxic agents, or obstruction. An abdominal ultrasound can provide initial imaging information, but further investigations are necessary to confirm the cause and guide management.