**Core Concept**
Lithium carbonate is a medication used to treat bipolar disorder, but its use during pregnancy can have teratogenic effects on the fetus. The most common congenital anomaly associated with lithium exposure in utero is a specific type of cardiac defect.

**Why the Correct Answer is Right**
Lithium can cause cardiac malformations, particularly Ebstein's anomaly, which is characterized by an abnormal formation of the tricuspid valve and right ventricle. This occurs due to the inhibition of sodium-potassium ATPase, leading to an increase in sodium levels in the cardiac tissue, which disrupts normal heart development.

**Why Each Wrong Option is Incorrect**
**Option A:** While lithium exposure is associated with an increased risk of cardiac malformations, other congenital anomalies like neural tube defects are not the most common association.

**Option B:** There is no strong evidence to suggest that lithium exposure is commonly linked to an increased risk of renal anomalies in the fetus.

**Option C:** While some medications can cause fetal growth restriction, lithium is not the most commonly associated medication with this condition.

**Clinical Pearl / High-Yield Fact**
It's essential for obstetricians and primary care physicians to discuss the risks and benefits of lithium therapy with pregnant women, and consider alternative treatments if possible, to minimize the risk of congenital anomalies.

**Correct Answer:** C. Ebstein's anomaly