Final answer:
Humans and some other animals cannot synthesize vitamin C due to an evolutionary loss of an enzyme, requiring them to intake vitamin C through their diet. Vitamin A must also be sourced from the diet in the form of carotenoids from plants, while a lack of niacin or tryptophan in the diet can lead to vitamin B3 deficiency.
Step-by-step explanation:
The question asked is about animals that cannot make their own vitamin C and the reasons behind this inability. Among humans and some other animals, there is a requirement for vitamin C because they lack the physiological mechanism to synthesize it on their own. This is due to the evolutionary loss of an enzyme called L-gulonolactone oxidase, which is necessary for the biosynthesis of vitamin C. Consequently, these animals must obtain vitamin C from their diet, with fruits and vegetables being important sources to prevent deficiency.
Vitamin A is another nutrient that animals cannot synthesize; they depend on provitamins such as carotenoids from plants, which are converted to the active vitamin in the body. Meanwhile, vitamins like niacin (vitamin B3) can be synthesized by some animals from the amino acid tryptophan, but insufficient dietary intake of either can still lead to a deficiency. Essential nutrients, such as certain vitamins, must be included in the diet since they are vital for health and cannot be produced by the body itself.