Surnames and Disease!

Okay, so I have two questions that are unrelated but they're for the same story so I thought why spam with multiple posts.

So, anyways, for a plot purpose of a story that I'm working on I need a father and son to have different last names. They are biologically related, and the son's in the father's custody and they have a good relationship any everything... So I can't think of a reason why they might have different last names, but if they did it would help my plot fall into place much more easily.

In the son's childhood his mother dies, and she was the only child of her parents, and kinda the last of the bloodline (even though this is set in modern times). So I was wondering, though it would be unusual, would it be out of the scope of realistic for the son to carry on her last name for her? Or can you think of any other reason why their last names might be different?

My second question is about fast acting, easily misdiagnosed diseases. You know those "silent killer" diseases. It's important to the plot that a woman dies, and that it is a relatively quick death, that could have been prevented by her doctor. I was thinking maybe breast cancer, but I'm not sure. It's hard to find anything on google about misdiagnosed quick-killing diseases that don't make the doctor who did the misdiagnosing compltely negligent. It's important that the doctor in the story is a good guy, and that it was a mistake, but one that he can easily (and somewhat rightfully) be blamed for.

Thanks so much in advance for all your help! =)

PS: The woman who dies would be between 37-40 years old, if that's important for what disease it could be.