**Core Concept**
The management of hyperthyroidism in pregnancy involves considering the effects of both the disease and its treatment on the mother and fetus. **Thyroid hormones** play a crucial role in fetal development, and their imbalance can have significant consequences. The treatment must balance the risk of uncontrolled hyperthyroidism against the potential teratogenic effects of antithyroid drugs.

**Why the Correct Answer is Right**
During the 2nd trimester of pregnancy, the drug of choice for hyperthyroidism is **Propylthiouracil (PTU)** due to its lower risk of teratogenicity compared to **Methimazole**. PTU inhibits the synthesis of thyroid hormones by blocking the enzyme **thyroid peroxidase**, which is involved in the conversion of iodide to iodine and the coupling of iodothyronine residues. This mechanism reduces thyroid hormone production without significantly affecting the fetal thyroid.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not specified, but typically, **Methimazole** is an alternative that might be considered due to its higher risk of teratogenicity in the first trimester.
**Option B:** Incorrect as it is not provided, but generally, other options like **Radioiodine** are contraindicated in pregnancy due to the risk of destroying the fetal thyroid gland.
**Option D:** Incorrect because, like Option B, it is unspecified but would likely represent another inappropriate choice for the 2nd trimester.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that **PTU** is preferred in the first and second trimesters due to the potential teratogenic effects of **Methimazole**, but due to the risk of **PTU**-induced hepatotoxicity, some guidelines suggest switching to **Methimazole** in the third trimester if possible.

**Correct Answer:** D. Propylthiouracil.