This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fluid and Electrolyte Treatment of Specific Disorders, Subject name: Pediatrics
The most sensitive indicator of depletion of intravascular volume in infant is: 
Select and explain. 
A. Cardiac output, B. Stroke volume, C. Heart rate, D. Blood pressure
C: Heart rate
Answer: C: Heart rate 
Explanation: c. Heart rate(Ref: Nelson's 20/e p 523-525, Ghai 8/e p 715-718)In infants with shock, compensatory increases in cardiac output are achieved primarily by "tachycardia" rather than by increase in stroke volume. Heart rates of 190-210/min are common in infants with compensated shock.