isobutyr-
combining form 
or isobutyro-
Etymology: International Scientific Vocabulary, from isobutyric (in isobutyric acid) 
 : isobutyric : related to isobutyric acid 
< isobutyramide >
< isobutyronitrile >
or isobutyro-
< isobutyramide >
< isobutyronitrile >