Your patients try your patience.
I would say death, no one can get used to that.
being killed
What is the most difficult thing about being a cowboy
the most interesting thing about being a doctor is that you can help people
The most challenging thing is surgery.
The language barrier is usually the most difficult aspect.
Survive being cast adrift when his crew mutinied. He apparently did not meet that challenge.
I think the most difficult thing Hillary Clinton face is to serve the nation while most people stone her with bad issues.
The Most important subject that you at least need to be good at for doctor in Biology and Chemistry these are the most important subjects for a doctor or a medical doctor (you need at least an A) . Doctors aren't that easy in fact it's very difficult
dough
A cockroach.
turning it on! lol
"most difficult" is a personal thing. What you find most difficult someone else might find easy and also, the other way around.
Create a perputual motion machine