Which among the following is the richest source of vitamin D?
**Core Concept**
Vitamin D is a fat-soluble vitamin crucial for bone metabolism, immune function, and calcium homeostasis. It is obtained through sunlight exposure, diet, and supplements. The richest sources of vitamin D vary among different food groups.
**Why the Correct Answer is Right**
Vitamin D3 (cholecalciferol) is the most abundant form of vitamin D in nature. Fatty fish, such as mackerel and sardines, are rich in vitamin D3 due to their diet and fatty tissue content. This vitamin D3 is then stored in the fatty tissues and organs of the fish, making it a significant source of this essential nutrient. The high fat content in these fish also enhances the bioavailability of vitamin D3.
**Why Each Wrong Option is Incorrect**
* **Option A:** While egg yolks do contain small amounts of vitamin D, they are not the richest source compared to fatty fish.
* **Option B:** Fortified milk and cereals may contain vitamin D, but the amounts are generally lower than those found in fatty fish.
* **Option D:** While sunlight exposure triggers the production of vitamin D in the skin, it is not a food source.
**Clinical Pearl / High-Yield Fact**
Remember that fatty fish, particularly mackerel and sardines, are the richest sources of vitamin D3. This is essential for patients with vitamin D deficiency or insufficiency, especially those with malabsorption syndromes or osteoporosis.
**Correct Answer:** C. Fatty fish.