[Publius]

That's the problem. Which name?!

[MineralMan]

"That's the problem. Which name?!
"

How about by the name the person prefers? I fail to see the problem, frankly. You're not courting this person. You're not planning to date this person. Why not just treat the person like everyone else in your office?

While you may have reservations about transsexuals, discourtesy to a co-worker is not an appropriate means of expressing them.

No doubt you know what name is preferred. Why not just use it and get on with things?