**Core Concept**
The question tests knowledge of antithyroid medications, specifically their onset of action. **Thyroid disorders** are commonly managed with these drugs, which inhibit thyroid hormone synthesis.

**Why the Correct Answer is Right**
The fastest acting antithyroid drug is not explicitly stated, but among common options, **propylthiouracil** and **methimazole** are well-known. However, when considering rapidity of action, particularly in the context of a **thyroid storm**, **iodine** or more specifically, **potassium iodide**, can be considered for its rapid inhibitory effects on thyroid hormone release.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option provided, it's challenging to address each directly, but generally, other antithyroid drugs like methimazole may have a slower onset compared to iodine's effects on thyroid hormone release.
**Option B:** Similar to Option A, without specifics, we can say that radioactive iodine, for example, has a slower onset of action due to its mechanism of destroying thyroid tissue over time.

**Clinical Pearl / High-Yield Fact**
In managing **thyrotoxic crisis**, it's crucial to remember that **iodine** (as potassium iodide) can rapidly decrease thyroid hormone release, acting within hours, making it a key component in the acute management of severe hyperthyroidism.

**Correct Answer:** D. Potassium iodide