**Core Concept**
The peripheral conversion of thyroxine (T4) to triiodothyronine (T3) is a critical step in thyroid hormone metabolism. This process is mainly catalyzed by the enzyme 5'-deiodinase, which is responsible for the conversion of T4 to T3. The regulation of this conversion is essential for maintaining normal thyroid hormone levels in the body.

**Why the Correct Answer is Right**
The correct answer is **Propylthiouracil (PTU)**. Propylthiouracil inhibits the peripheral conversion of T4 to T3 by blocking the activity of 5'-deiodinase. This is achieved through a mechanism that involves the inhibition of the thyroid hormone-binding globulin (TBG) and the reduction of T4 to T3 conversion. PTU is a medication used to treat hyperthyroidism, and its ability to inhibit T4 to T3 conversion makes it an effective treatment option.

**Why Each Wrong Option is Incorrect**
* **Option A:** Methimazole is an antithyroid medication that inhibits thyroid hormone synthesis, but it does not affect the peripheral conversion of T4 to T3.
* **Option B:** Iodine is essential for thyroid hormone synthesis, but it does not inhibit the peripheral conversion of T4 to T3.
* **Option C:** Furosemide is a diuretic medication that does not have any effect on thyroid hormone metabolism or the peripheral conversion of T4 to T3.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Propylthiouracil (PTU) is a medication that can inhibit the peripheral conversion of T4 to T3, making it a useful treatment option for hyperthyroidism. However, PTU can cause liver damage and should be used with caution.

**Correct Answer:** C. Furosemide