This is Begonia U521, and unidentified species from Thailand. When begonias are newly discovered, but not named or recognized, they are given a sequential unidentified number. The American Begonia Society is in charge of these listings.
This rhizomatous begonia gets quite large, as my specimen is outgrowing a 12" pot, and has nice light pink flowers on upright branches. It is quite beautiful, and the darkness of the green is amazing against the silvery patterns.