Among the following patient scenarios, which one is at highest risk specifically for vitamin B12 deficiency and which preventive measure is most appropriate?

A long-term strict vegan who avoids all animal products — recommend routine B12 supplementation or fortified foods because B12 is almost exclusively found in animal-derived foods and may take years to deplete.

A breastfeeding mother who consumes fish daily — recommend stopping fish due to it causing immediate B12 depletion in the infant.

A middle-aged individual who eats meat daily but exercises strenuously — recommend iron supplementation as the primary prevention for B12 deficiency.

A teenager on a high-citrus fruit diet — advise adding green vegetables because citrus fruits block B12 absorption quickly.