**Core Concept**
Lithium carbonate is a medication used to treat bipolar disorder, and its use during pregnancy can have teratogenic effects on the developing fetus. The most common anomaly associated with lithium exposure in utero is related to cardiac development.

**Why the Correct Answer is Right**
The cardiac anomalies associated with lithium exposure are thought to be due to the inhibition of the sodium-potassium ATPase pump in cardiac myocytes, leading to an increase in the intracellular sodium concentration. This disrupts the normal functioning of the cardiac sodium-calcium exchanger, resulting in an influx of calcium ions into the cell. The increased calcium levels trigger an abnormal contraction of the cardiac muscle, leading to cardiac deformities. The most common cardiac anomalies seen in association with lithium exposure are Ebstein's anomaly, a congenital heart defect characterized by an abnormal formation of the tricuspid valve and right ventricle.

**Why Each Wrong Option is Incorrect**
**Option B:** Neural tube defects are associated with folic acid deficiency, not lithium exposure.
**Option C:** Limb reduction defects are associated with thalidomide exposure, not lithium.
**Option D:** Genitourinary deformities are associated with various maternal conditions, such as diabetes and urinary tract infections, but not specifically with lithium exposure.

**Clinical Pearl / High-Yield Fact**
It is essential for pregnant women taking lithium to be closely monitored by their healthcare provider, as the risk of cardiac anomalies in the fetus increases with the duration of lithium exposure.

**✓ Correct Answer: A. Cardiac deformities**