Cornus canadensis; Cornaceae (dogwood family)
Now I’m cheating. According to the USDA, bunchberry can be found in Maryland. But it’s a northern plant that doesn’t like the heat and humidity of mid-Atlantic piedmont summers (I should know, I’ve tried growing it often enough), so you won’t find it in the Potomac gorge. You might be able to find it in the westernmost part of the state (Garrett County), at the higher elevations. Or maybe not; it’s endangered there, as well as in Indiana and Illinois. And it’s threatened in Iowa and Ohio.
This one I found on Cape Breton Island, Nova Scotia, in September (very unusual for it to be blooming so late in the year), in the boreal forest region.
Really looks like our common flowering dogwood (Cornus florida), doesn’t it? But it isn’t a tree or shrub; it’s a groundcover, standing less than a foot high.