Which vitamin deficiency is likely present in a cat exhibiting ventral neck flexion on a fish-only diet?

The cat exhibiting ventral neck flexion on a fish-only diet is most likely suffering from a deficiency in Vitamin B1, also known as thiamine. A fish-only diet can lead to this deficiency because certain fish contain thiaminase, an enzyme that breaks down thiamine, effectively rendering it unavailable for absorption.

Thiamine is essential for several metabolic processes, including carbohydrate metabolism and energy production. A deficiency can result in neurological issues, including the observed ventral neck flexion, which is indicative of neurological dysfunction. In cats, this condition is often referred to as "thiamine deficiency syndrome," and it can manifest with other signs such as ataxia, seizures, and lethargy.

In contrast, while Vitamin B12, Vitamin A, and Vitamin D are important nutrients for various physiological functions, the specific neurological symptoms seen in this scenario are most closely associated with thiamine deficiency. Each of these other vitamins plays distinct roles in the body, but they are not typically linked to the characteristic clinical signs observed with thiamine deficiency in cats fed a diet deficient in this essential nutrient.