Viewing hidden photos within a specific album

It is possible on a Mac with a smart album, but not on the iPhone, because smart albums are not supported on the iPhone.

You may want to use a work-around and use password protected notes for the sensitive photos you want to show to your doctor. In Notes you can create a folder "Doctor" where you collect notes with sensitive photos for easy access. Then lock the notes with a password. You will have to enter this password before you can open a locked note.

How to lock notes is described here: Lock your notes on iPhone - Apple Support

And here how to organize the notes using folders: Organize your notes in folders on iPhone - Apple Support

It's a bit awkward, but try this:

1. Select all the pictures in "Hidden" and add them to a new album called MyHidden. The new album will look empty, of course, but this will keep you from losing track of them.
2. In "Hidden," select the medical pictures and add them to a new album called Medical. This album will look empty, too.
3. When you're at the doctor, unhide the pictures in Hidden. Then you can see the pictures in Medical
4. When you're through, go to My Hidden, and hide them all, again.

If it's important to you to have only Medical pictures unhidden even for a short time, then you could remove the Medical pictures from MyHidden so they only exist in Medical and Hidden. Then you could either

1. Unhide only the Medical pictures, if they're easy to collect in Hidden, or
2. Unhide all the pictures and then immediately hide MyHidden (which wouldn't include Medical.)
3. Then re-hide Medical when you're through.

What do you think?