Nice explanation. Thanks, David. I went back to the German version, and my translation there was 'fixed idea.' Similar.

Isn't fiks idé exactly from the french "idée fixe"? This takes me right back to the music conservatory studying Richard Wagner operas and his melodic motifs that were described as "idée fixe" meaning a motif that recurs in the music in association with a particular character or theme of the story. It has often been used since then to describe a psychological obsession with one thought that keeps recurring and bothering a person. I think the English reader should be familiar with that use of the french term and therefor we should use the french term: <<That is often the case with people who carry an "idée fixe".>> Or instead of "carry" say <<...who are burdened with an "idée fixe">> " The verb bærer på seems to fit "burdened with".