**Core Concept**
Thyrotoxicosis during pregnancy requires careful management due to the risks of maternal and fetal complications. The goal is to control hyperthyroidism while minimizing the risk of adverse effects on the fetus.

**Why the Correct Answer is Right**
The drug of choice for treating thyrotoxicosis during pregnancy is **Propylthiouracil (PTU)**. PTU is preferred because it crosses the placenta less than methimazole, reducing the risk of antithyroid drug-induced congenital anomalies. PTU also has a more rapid onset of action, making it a better choice for managing acute thyrotoxic crisis.

**Why Each Wrong Option is Incorrect**
**Option A:** Methimazole - Methimazole has a higher risk of causing congenital anomalies, such as esophageal atresia and choanal atresia, making it a less desirable choice during pregnancy.
**Option B:** Iodine - Iodine is not a first-line treatment for thyrotoxicosis due to its potential to exacerbate hyperthyroidism in some cases.
**Option C:** Beta-blockers - Beta-blockers, such as propranolol, are used to manage symptoms of hyperthyroidism, such as tachycardia and tremors, but they do not treat the underlying cause of thyrotoxicosis.

**Clinical Pearl / High-Yield Fact**
In pregnant women with thyrotoxicosis, it's essential to use an antithyroid medication that minimizes the risk of congenital anomalies while effectively controlling hyperthyroidism. PTU is the preferred choice due to its lower risk of teratogenicity.

**Correct Answer: D. Propylthiouracil**