**Core Concept**
Polyhydramnios is a condition characterized by an excessive accumulation of amniotic fluid, which can be caused by various factors including fetal anomalies, maternal diabetes, and Rh incompatibility. The underlying pathophysiology involves an imbalance between fetal urine production and its absorption by the amniotic membranes.

**Why the Correct Answer is Right**
In a normal pregnancy, the amount of amniotic fluid is maintained by a balance between fetal urine production and its absorption by the amniotic membranes. However, in cases of polyhydramnios, this balance is disrupted, leading to an accumulation of excess fluid. Fetal anomalies such as esophageal atresia or duodenal atresia can cause polyhydramnios by impairing fetal swallowing and thereby reducing the amount of amniotic fluid absorbed.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because maternal diabetes can cause polyhydramnios by increasing fetal urine production.
**Option B:** This option is incorrect because Rh incompatibility can cause polyhydramnios by leading to fetal anemia and increased fetal urine production.
**Option C:** This option is incorrect because fetal anomalies such as esophageal atresia or duodenal atresia can cause polyhydramnios by impairing fetal swallowing and thereby reducing the amount of amniotic fluid absorbed.

**Clinical Pearl / High-Yield Fact**
Fetal anomalies such as esophageal atresia or duodenal atresia are common causes of polyhydramnios, and their presence should be suspected in cases of excessive amniotic fluid accumulation.

**Correct Answer: D.**