Sanchez

Doctor Sanchez was a 23rd century Human Starfleet medical examiner who served aboard the USS Enterprise under Captain James T. Kirk during the mid-2260s.

In 2268, Dr. Sanchez conducted an autopsy on Ensign Wyatt after he was killed by Losira. He discovered that the cause of death was cellular disruption - as if "each cell had been individually blasted from the inside", and ruled out a pathogen as the cause of this disruption. (TOS: "That Which Survives")

Sanchez was only briefly seen behind Dr. M'Benga in an adjacent room conducting the autopsy.
He was played by an unknown actor.