**Core Concept**
Iodine is an essential micronutrient required for the synthesis of thyroid hormones, particularly triiodothyronine (T3) and thyroxine (T4), which play crucial roles in regulating metabolism, growth, and development in humans.

**Why the Correct Answer is Right**
Iodine is required in humans to prevent iodine deficiency disorders (IDD), including goiter, hypothyroidism, and cretinism. The recommended daily intake of iodine is approximately 150 micrograms (mcg) for adults, which can be obtained through iodized salt, seaweed, and dairy products. Iodine is absorbed in the gut and transported to the thyroid gland, where it is oxidized and incorporated into thyroglobulin, a protein that serves as a precursor for the synthesis of T3 and T4.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because iodine is not a necessary nutrient for humans in the context of maintaining healthy skin and bones. While iodine is essential for thyroid function, its primary role is not related to skin or bone health.

**Option B:** This option is incorrect because the recommended daily intake of iodine is not 50 mcg. The recommended daily intake is actually around 150 mcg for adults.

**Option C:** This option is incorrect because iodine is not typically obtained through drinking water. While some drinking water sources may contain iodine, it is not a reliable source of iodine for humans.

**Clinical Pearl / High-Yield Fact**
Iodine deficiency is a significant public health concern, particularly in areas where iodized salt is not readily available. The World Health Organization (WHO) recommends that all salt be iodized to prevent IDD.

**Correct Answer:** D.