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: Physiology
In quiet breathing, and in the supine position, where is the apex beat of the hea located? 
Select and explain. 
A. To the left of the sternal angle at the level of the second rib, B. In the left fifth intercostal space in the midclavicular line, C. In the right midaxillary line, D. At the xiphisternal junction
B: In the left fifth intercostal space in the midclavicular line
Answer: B: In the left fifth intercostal space in the midclavicular line 
Explanation: The apex is the only point of the hea that can be directly identified on the precordium. A cardiac impulse may be visible at the apex, and palpation over it will confirm the presence of the apex beat. The apex is located in the left fifth intercostal space just medial to the midclavicular line. Ref: Ommen S.R., Nishimura R.A., Tajik A. (2011). Chapter 33. Hyperophic Cardiomyopathy. In V. Fuster, R.A. Walsh, R.A. Harrington (Eds), Hurst's The Hea, 13e.