Darnell

From Memory Alpha, the free Star Trek reference

Mr. Darnell was a sciences division crewman aboard the USS Enterprise serving under Captain James T. Kirk in 2266.

Darnell was among the original landing party, along with Kirk and McCoy, to beam down to planet M-113. The landing party was there to perform the routine medical checks required of all scientific expeditions. The image of Nancy Crater, as Darnell saw her, reminded him of someone he had left behind on Wrigley's Pleasure Planet.

While on the planet, Darnell became the first victim of the M-113 creature. His autopsy revealed that all of the salt was suddenly depleted from his body, a condition that was medically impossible by any standards. (TOS: "The Man Trap")

Darnell was played by the late actor Michael Zaslow. He was the first person pronounced dead on-screen by Dr. McCoy