This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Physiology
What is the physiological dead space when aerial PCO2 (PaCO2) is 48mmHg, expired PCO2 (PECO2) is 25mmHg and tidal volume of 500ml? 
Select and explain. 
A. 260ml, B. 240ml, C. 220ml, D. 200ml
A: 260ml
Answer: A: 260ml 
Explanation: Bohr's equation PCO2 x VT = PCO2 x (VT - PDS)25 x 500 = 48 (500 - PDS)12500/48=500 - PDS260 =500-PDSPDS = 240ml