This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart, Circulation, and Blood, Subject name: Physiology
The lead of ECG marked as X is called? 
Select and explain. 
A. Lewis lead, B. V4R, C. aVR, D. Vg
B: V4R
Answer: B: V4R 
Explanation: And. B. V4R15 - Lead ECGV4R - 5th intercostal space midclavicular line (same as V4 but on the other side)V8 - On the back, 5th intercostal space midscapular line.V9 - On the Back, 5th intercostal space between V8 and the spine