Diagnose

google

ref

mid 19th century: back-formation from diagnosis.

wiktionary

ref

Back-formation from  diagnosis. 

etymonline

ref

diagnose (v.)

"to ascertain or determine (a disease) from its symptoms," 1861, back-formation from diagnosis (q.v.) on the model of metamorphose, etc. Earlier was diagnosticate (by 1834). Related: Diagnosed; diagnosing.