What is the primary mechanism of action of neomycin when used to treat hepatic encephalopathy?

Neomycin acts primarily by killing bacteria in the gut that produce ammonia, which is a key factor contributing to hepatic encephalopathy. In patients with liver dysfunction, the conversion of nitrogenous compounds to ammonia by gut bacteria elevates blood ammonia levels, leading to neurotoxicity and encephalopathy. By administering neomycin, the growth of these ammonia-producing bacteria is suppressed, resulting in decreased ammonia production. This reduction helps to lower overall ammonia levels in the bloodstream and alleviates the symptoms associated with hepatic encephalopathy.

The other mechanisms listed do not directly explain the primary action of neomycin in this context. For instance, while binding of ammonia in the gut may be relevant to other treatments like lactulose, it does not accurately describe neomycin's function. Likewise, the reduction of new protein synthesis and blocking metabolism of folic acid do not pertain to neomycin’s antibacterial effects or its role in managing ammonia levels. Thus, the primary mechanism is indeed the antibacterial action against ammonia-producing gut flora.