DrScheme supports several historically useful variants of Scheme without a #lang prefix:
The R5RS language contains those primitives and syntax defined in the R5RS Scheme standard. See the r5rs library for details.
The PLT Pretty Big language provides a language roughly compatible with a language in earlier versions of DrScheme. It evaluates a program in the same way as load, and it starts by importing the following modules: mzscheme, scheme/gui/base, mzlib/class, mzlib/etc, mzlib/file, mzlib/list, mzlib/unit, mzlib/include, mzlib/defmacro, mzlib/pretty, mzlib/string, mzlib/thread, mzlib/math, mzlib/match, and mzlib/shared.