**Core Concept**
Topical steroids are a class of corticosteroids that are used to reduce inflammation and suppress the immune system in various skin conditions. They vary in potency, with higher potency steroids causing more pronounced side effects.

**Why the Correct Answer is Right**
The potency of topical steroids is determined by their ability to penetrate the skin and reach the site of inflammation. Hydrocortisone is the least potent topical steroid, as it has a molecular weight that makes it difficult for it to penetrate the skin, limiting its effectiveness. This makes it a safer option for long-term use, but less effective for severe skin conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Dexamethasone is a potent topical steroid that is often used for severe skin conditions, making it incorrect as the least potent option.
**Option B:** Betamethasone is another potent topical steroid that is commonly used for a variety of skin conditions, making it an incorrect choice.
**Option C:** Triamcinolone is a potent topical steroid that is often used for severe skin conditions, making it an incorrect choice.

**Clinical Pearl / High-Yield Fact**
When choosing a topical steroid, it's essential to consider the potency and potential side effects. Hydrocortisone is a good starting point for mild skin conditions, but more potent steroids may be necessary for severe conditions.

**Correct Answer: D. Hydrocortisone**