Explain why the combination of disturbed grammar and paraphasic errors together is considered a particularly distinguishing feature of central aphasia.

The presence of paraphasic errors alongside disturbed grammar is a distinguishing combination of features, because either feature alone could plausibly arise from a narrower, more specific disturbance: paraphasic errors alone might suggest a language-processing problem confined to word selection, while disturbed grammar alone might suggest a more purely expressive difficulty. Seeing both together, along with impaired comprehension and impaired writing as well as speech, points instead toward a more substantial, combined disturbance affecting both understanding and production, which is precisely what distinguishes central aphasia from the narrower, single-function patterns seen in nominal aphasia or the purely receptive or purely expressive categories.