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
Earliest symptom of acute mountain sickness is 
Select and explain. 
A. Blurring of vision, B. Fever, C. Nausea and vomiting, D. Headache
D: Headache
Answer: D: Headache 
Explanation: The low paial pressure of oxygen at high altitude causes aeriolar dilatation and if cerebral autoregulation does not compensate, there is an increase in capillary pressure that ors increased transudation of fluid into brain tissue.Ref: Ganong's Review of Medical Physiology Kim E Barrett, Susan M Barman, Scott Boitano, Heddwen L Brooks 23rd edition page no - 619