<prompt> Earliest symptom of acute mountain sickness isBlurring of visionFeverNausea and vomitingHeadache  <response>: 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: Ganongs Review of Medical Physiology Kim E Barrett, Susan M Barman, Scott Boitano, Heddwen L Brooks 23rd edition page no - 619 