**Core Concept**
Graves' disease is an autoimmune disorder causing hyperthyroidism, characterized by excessive production of thyroid hormones (T3 and T4). In pregnancy, the preferred treatment aims to control hyperthyroidism while minimizing risks to the fetus.

**Why the Correct Answer is Right**
During pregnancy, the goal is to achieve euthyroidism (normal thyroid hormone levels) to prevent maternal and fetal complications. Methimazole (MMI) is the preferred antithyroid drug in the first trimester due to its efficacy and lower risk of congenital anomalies compared to radioactive iodine (RAI) or propylthiouracil (PTU). MMI inhibits thyroid hormone synthesis by blocking the enzyme thyroid peroxidase (TPO).

**Why Each Wrong Option is Incorrect**
* **Option A:** Radioactive iodine (RAI) is contraindicated in pregnancy due to its potential to cross the placenta and ablate the fetal thyroid gland, leading to permanent hypothyroidism.
* **Option B:** Propylthiouracil (PTU) is generally avoided in the first trimester due to its potential to cause liver damage and is preferred in the second and third trimesters.
* **Option C:** Subtotal thyroidectomy is a surgical option for hyperthyroidism but is not preferred in pregnancy due to the risks associated with surgery and the availability of effective medical treatments.

**Clinical Pearl / High-Yield Fact**
In pregnant women with Graves' disease, methimazole is the preferred antithyroid drug in the first trimester, and propylthiouracil is generally reserved for the second and third trimesters.

**Correct Answer:** A. Methimazole.