This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
The most common early sign of kidney disease is: 
Select and explain. 
A. Sodium retention, B. Elevated BUN level, C. Development of metabolic acidosis, D. Inability to dilute or concentrate urine
B: Elevated BUN level
Answer: B: Elevated BUN level 
Explanation: Increased BUN is usually an early indicator of decreased renal function.