The prosopagnosic patient PS having to name fruits and vegetables, one by one. She can do that correctly and rapidly, despite the visual similarity between items of this category. Such observations contradict the view that prosopagnosia is an impairment at recognizing/discriminating visually similar items.
See:
Busigny, T., Graf, M., Mayer, E., Rossion, B. (2010). Acquired prosopagnosia as a face-specific disorder: Ruling out the general visual similarity account . Neuropsychologia, 48, 2051-2067. [PDF]