“You’ll start your first incision at the superior margin of the sternum and take it down to about the umbilicus.”
A group of first-year medical students dressed in white scrubs crowded around the professor and, of course, the cadaver. The white plastic sheet that had covered the body a moment ago had been pulled back to expose its torso. The words “Male, 52 COD: Cardiopulmonary Failure” were written on the sheet in permanent black ink near its legs.
“Don’t cut too deep,” the professor continued. “Just through the skin; we don’t want to damage the tissue beneath.”
Mark saw none of this, but he heard it. And when the scalpel began to cut into his chest, he felt it.
By reflex, he tried to lift his arms and grab at the scalpel, but

It might be more powerful in 1st person (or 3rd limited and starting out just in his head) as it would allow us to experience the world inside Mark's eyes; the confusion, darkness, the noises of people entering the room, small conversations and objects being moved around, the cloth coming off, the feeling of being naked and cold, the lecture from the teacher and finally the sharp pain of something cutting into his chest.

It's an interesting opening and I would definitely read on, though I think it could be tightened up a little:

A group of first-year medical students dressed in white scrubs crowded around the professor and, of course, the cadaver. The white plastic sheet that had covered the body a moment ago had been pulled back to expose its torso. Might read better as: "A group of first-year medical students crowded around the cadaver. The white plastic sheet had been pulled back to expose its torso." Just an idea.

And when the scalpel began to cut into his chest, he felt it. But then I thought this was underwritten. He felt it? What did he feel? Blazing agony or a slight tugging on his skin? We aren't familiar enough with the MC yet to know how he would react to this, I thought showing more of what he felt might get the reader's sympathy for him early in the story, to identify with him more.

By reflex, he tried to lift his arms and grab at the scalpel, but I'd cut "By reflex", I don't think it adds anything to the sentence here

Looks good, I'll be glad to give it a read if you like.

I think you're right, halogen. I considered writing it in first person, thinking it would be more intense, but settled on 3rd because I wanted the reader glimpse the scene from the outside. Now that it is done (and short) I might redo some or all of it in 1st to see how it goes. But either way, I think your suggestions for adding some detail here are good ones.

Martin, I agree that the white scrubs are implied, and I can omit mention of the professor since we already know he is there. The words in black permanent ink have meaning to me, but the reader can probably do without them. Of course, if I switch to first person, it all goes away.

BTW, after posting last night, I started reading "Fledgling" by Octavia Butler. The main character wakes up in the dark and in pain not knowing where she is... I learned a lot from her first person rendition of a similar situation.

Jon.

When I woke up, I couldn’t move and I couldn’t see. I was naked, except for what felt like a rough sheet covering me from the waist down. I was lying on my back on something cold and hard.
I realized that I wasn’t breathing. I tried to inhale, but nothing happened. My chest was frozen in place.
Just as my fear began to turn to panic, I heard talking.
“You’ll start your first incision at the superior margin of the sternum and take it down to about the umbilicus.”
It sounded like a professor lecturing.
“Don’t cut too deep,” he continued. “Just through the skin; we don’t want to damage the tissue beneath.”
There was an incredible, sharp pain in my chest. I tried to lift

Think:
At first the voices were faint-- surreal.

As I became aware.

Scratch the waking up.

3rd limited includes: Cinematic, deep penetration, light penetration...but not omniscient.

Also if Mark is your POV character then you need to establish that straight away. I tend to take a cue from the first person doing something or saying something and make the assumption they are the POV character. You didn't correct this.

e.g. Mark listened to the professor as he addressed the students...

That establishes him as the POV character.

I realize you want a reveal that the POV guy is a corpse (or a sushi eater) but you can do that in a number of ways anyway.

The prose is good, better in the second version, but I still reckon 3rd limited is the way to go.