**Core Concept**
Polyhydramnios is a condition characterized by an excessive amount of amniotic fluid surrounding the fetus, typically defined as an amniotic fluid index (AFI) of ≥24 cm. It can be associated with various fetal and maternal factors that impair normal fetal swallowing or maternal glucose metabolism.

**Why the Correct Answer is Right**
Polyhydramnios is often caused by conditions that either reduce fetal swallowing or impair maternal glucose metabolism. Fetal swallowing is responsible for removing excess amniotic fluid from the fetal lungs and gastrointestinal tract. When fetal swallowing is impaired, the amount of amniotic fluid can increase. Maternal diabetes mellitus, a condition characterized by high blood glucose levels, can also contribute to polyhydramnios. This is because high glucose levels in the maternal bloodstream can lead to fetal macrosomia and increased fetal glucose levels, stimulating fetal insulin production and subsequent fetal growth.

**Why Each Wrong Option is Incorrect**
* **Option A:** Maternal diabetes mellitus is actually a cause of polyhydramnios, so this option is incorrect because it is a known risk factor.
* **Option B:** Fetal anemia can lead to non-immune hydrops fetalis, a condition characterized by excessive fluid accumulation in the fetus, but it is not a direct cause of polyhydramnios.
* **Option C:** Maternal hypertension is not a direct cause of polyhydramnios, although it can contribute to fetal growth restriction and other complications.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the causes of polyhydramnios is "DIABETES": Diabetes (maternal), Anencephaly (fetal), Beta-thalassemia (fetal), Erythroblastosis fetalis (fetal), Insulin resistance (maternal), Thyroid disease (maternal), Erythrocytosis (maternal), and Spinal dysraphism (fetal).

**Correct Answer: A. Maternal diabetes mellitus is actually a cause of polyhydramnios.**