This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Oncology, Subject name: Medicine
A 21-year-old woman presents with a 2-day history of abdominal pain, nausea, and vomiting. On examination, her blood pressure is 100/70 mmHg, pulse 100/min, JVP is below sternal angle, and abdomen is diffusely tender. Her urine output is low and electrolytes reveal elevated urea and normal creatinine.For the above patient with AKI, select the characteristic urine electrolyte and microscopy findings. 
Select and explain. 
A. urinary osmolality < 100, B. oxalate crystals in the urine, C. urinary sodium < 20, D. fractional sodium excretion > 1%
C: urinary sodium < 20
Answer: C: urinary sodium < 20 
Explanation: In prerenal AKI (ie, no histologic damage), the urinalysis reveals only hyaline casts. The urine sodium is low (< 20), the urinary osmolality is high (> 500), and the fractional excretion of sodium is < 1.