monomaniacal

noun

1. (no longer in technical use) a psychosis characterized by thoughts confined to one idea or group of ideas.
2. an inordinate or obsessive zeal for or interest in a single thing, idea, subject, or the like.

noun

1. an excessive mental preoccupation with one thing, idea, etc

n.“insanity in regard to a single subect or class of subjects,” 1820, probably on model of earlier French monomanie, from Modern Latin monomania, from Greek monos “single, alone” (see mono-) + mania (see mania). n.

1. Pathological obsession with one idea or subject, as in paranoia.