Key Concept: Freshwater fish diseases are grouped by causative organism — bacterial, parasitic, or protozoan.
Why it's correct: Columnaris, caused by the bacterium Flavobacterium columnare, is the most common bacterial disease in freshwater aquaculture, producing lesions and ulcers on gills, fins, and skin.
Why others fail: Argulosis (a parasitic crustacean), Ichthyophthiriasis (a protozoan), and Dactylogyrosis (a monogenean flatworm) are all non-bacterial diseases.