**Core Concept**
Carbimazole is a thionamide antithyroid medication that inhibits thyroid hormone synthesis by blocking the enzyme thyroperoxidase. This leads to a reduction in the production of thyroid hormones, making it an effective treatment for hyperthyroidism, including thyrotoxicosis in pregnancy.

**Why the Correct Answer is Right**
In pregnancy, the use of antithyroid medications like carbimazole is preferred over radioactive iodine due to its safety profile for the fetus. Carbimazole crosses the placenta but is primarily metabolized by the fetus, allowing for a lower maternal dose and reducing the risk of fetal goiter or hypothyroidism. Additionally, carbimazole is effective in controlling hyperthyroidism symptoms, such as tachycardia and weight loss, which can be detrimental to both the mother and the fetus.

**Why Each Wrong Option is Incorrect**

* **Option B:** Lugol's iodine is used in the treatment of thyrotoxic crisis, but it is not the drug of choice for managing thyrotoxicosis in pregnancy. Lugol's iodine can actually worsen fetal goiter and hypothyroidism due to its high iodine content.
* **Option C:** Propranolol is a beta-blocker used to control the symptoms of hyperthyroidism, such as tachycardia and tremors. While it can be used in pregnancy, it does not address the underlying cause of hyperthyroidism and is not a substitute for antithyroid medications like carbimazole.
* **Option D:** Radioactive iodine is contraindicated in pregnancy due to its potential to cross the placenta and cause fetal thyroid ablation, leading to permanent hypothyroidism.

**Clinical Pearl / High-Yield Fact**
It's essential to note that antithyroid medications like carbimazole require careful monitoring of thyroid function tests and fetal well-being during pregnancy. Regular follow-up with an endocrinologist or obstetrician is crucial to ensure the safe management of hyperthyroidism in pregnant women.

**✓ Correct Answer: A. Carbimazole**