What does nitrite toxicity often lead to in fish?

Nitrite toxicity in fish primarily leads to methemoglobinemia, a condition where hemoglobin in the blood is oxidized to methemoglobin, which cannot effectively carry oxygen. This occurs when nitrite is absorbed into the bloodstream and interacts with hemoglobin, converting it to a form that reduces its oxygen-carrying capacity. As a result, fish can experience symptoms such as lethargy, difficulty in breathing, and overall hypoxia due to insufficient oxygen in the body, which can ultimately lead to severe health issues or death if not addressed promptly.

The other conditions listed do not typically arise directly from nitrite toxicity in fish. Exophthalmia (bulging eyes), while it can occur in fish due to various factors, is not a specific result of nitrite toxicity. Neurologic signs are associated with different types of stressors or diseases, but are not a direct consequence of nitrite exposure. Granulomatous plaques and lesions are often indicative of infectious processes or chronic inflammatory responses, rather than the acute impacts seen with nitrite toxicity. Therefore, the direct effect of nitrite on the blood's ability to transport oxygen makes methemoglobinemia the correct answer concerning nitrite toxicity in fish.