Hm. Under what circumstances does she misspell words? Is there a difference between her oral spelling, handwritten spelling, and typed spelling? Does she make other writing mechanics errors--punctuation, capitalization, letter formation, spacing? Can she identify and orally correct misspellings when she sees them? Spontaneously? When told to find the unspecified error?
When you spell a long complex word, most people (even very bright children) tend to slow down and think through the word, either using phoneme segmentation (hence, sounding it out) or morphologically (using word roots and affixes). For short, high-frequency words, one tends to rely on automatic skills, which, in the case of someone with ADHD, may be more vulnerable to inconsistent attention or self-monitoring.
I'll also note on the side that "toxoplasmosis" is actually quite amenable to using phonics, as, although two letters are present in more than one of their sounds, it has no exceptions in it.
