When some has the need to us the restroom may he answer “Amen” upon they hear a bracha?
The Rema (84:1) rules that one must not answer “amen” in a bathhouse. Although “amen” is not a davar she-bikdushah, and not a pasuk, it apparently “incorporates” the berachah, and causes one to think of the berachah (see Biur Halachah 588, s.v. shema).
The same will apparently apply to answering “amen” when needing to use the restroom (for an urgent need; see Shulchan Aruch 92:1; Biur Halachah).
Just as when “external conditions” are inapppropriate one does not answer amen, so when “internal conditions” are inappropriate for a berachah, the principle applies even for “amen.”