**Core Concept**
Lithium is a medication used to treat bipolar disorder, but its use during pregnancy can have significant effects on the developing fetus. This is due to its ability to cross the placental barrier and affect fetal development.

**Why the Correct Answer is Right**
Lithium can cause fetal goiter (enlargement of the thyroid gland) and hypothyroidism (underactive thyroid gland) due to its inhibition of thyroid hormone release from the thyroid gland. This is because lithium increases the expression of the sodium-iodide symporter in the thyroid gland, which leads to an accumulation of iodine and subsequent thyroid hormone suppression.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because lithium does not directly affect the fetal heart rate or rhythm. While lithium can have teratogenic effects, cardiac anomalies are not typically associated with its use during pregnancy.
**Option B:** This option is incorrect because lithium does not directly cause fetal growth restriction. While preterm birth and low birth weight are potential complications of lithium exposure, growth restriction is not a direct effect of the medication.

**Clinical Pearl / High-Yield Fact**
It is essential for pregnant women taking lithium to be closely monitored by their healthcare provider to minimize the risk of fetal goiter and hypothyroidism. Regular thyroid function tests and fetal ultrasound assessments can help identify potential issues early on.

**Correct Answer: C. Fetal goiter and hypothyroidism.**