What deficiency is likely to cause macrocytic anemia when switching from cow's milk to goat's milk?

The presence of macrocytic anemia when switching from cow's milk to goat's milk is often associated with a deficiency in folate. Goat's milk lacks sufficient amounts of folate, which is essential for DNA synthesis and the proper maturation of red blood cells. When an individual consumes a diet that lacks adequate folate, they can develop macrocytic anemia characterized by the production of larger-than-normal red blood cells that are ineffective in carrying oxygen.

In the context of dietary changes, cow's milk is typically fortified with vitamins and minerals, including folate, whereas goat's milk may not provide enough of these nutrients. This situation is particularly relevant for infants and young children whose primary nutrition comes from milk. As they transition to goat's milk, the insufficient folate intake can lead to the aforementioned anemia.

Other deficiencies, such as vitamin B12 and iron, may also cause anemia but are less specifically linked to this dietary transition from cow's milk to goat's milk. Vitamin K plays a role primarily in blood clotting and is not directly related to the development of macrocytic anemia. Thus, the deficiency most likely causing macrocytic anemia in this scenario is folate.