Final answer:
Vitamin C is the vitamin that all mammals except humans and guinea pigs can synthesize. It is a water-soluble vitamin essential for many bodily functions but must be obtained through diet for humans and guinea pigs.
Step-by-step explanation:
The vitamin that can be synthesized by all mammals except humans and guinea pigs is Vitamin C. Humans and guinea pigs lack the enzyme L-gulonolactone oxidase which is necessary for the synthesis of Vitamin C, and therefore, they need to obtain it from their diet. It is important to note that Vitamin C is a water-soluble vitamin, which unlike fat-soluble vitamins, is not stored in the body's lipids but is excreted from the body if ingested in excess.
Vitamin A is obtained from dietary sources, particularly from compounds like β-carotene found in orange vegetables, which the body can convert into the active form of the vitamin. Vitamin D synthesis occurs in the skin when exposed to UV radiation from sunlight, though it is also commonly fortified in foods. Vitamins are essential nutrients required in small quantities for a variety of bodily functions.