**Core Concept**
Spinal muscular atrophy (SMA) is a genetic disorder characterized by the degeneration of spinal motor neurons, leading to muscle weakness, wasting, and hypotonia in infants. It is often diagnosed prenatally or postnatally due to its association with polyhydramnios and decreased fetal movements.

**Why the Correct Answer is Right**
The correct answer, Spinal muscular atrophy (SMA), is right because it is a congenital condition that affects the motor neurons, causing muscle weakness and wasting. SMA is caused by mutations in the survival motor neuron 1 (SMN1) gene, leading to a deficiency of the survival motor neuron protein. This protein is essential for the survival of motor neurons. The prenatal history of polyhydramnios and decreased fetal movements is a classic presentation of SMA, as the fetus is unable to swallow amniotic fluid properly, leading to an accumulation of fluid in the womb. Additionally, the decreased fetal movements are due to the muscle weakness caused by the degeneration of motor neurons.

**Why Each Wrong Option is Incorrect**

* **Option B:** Classical myotonic dystrophy is a genetic disorder that affects muscle tone and is characterized by myotonia, muscle stiffness, and wasting. However, it does not typically present with polyhydramnios or decreased fetal movements, and it is more commonly diagnosed in later childhood or adolescence.
* **Option C:** Duchenne muscular dystrophy is a genetic disorder that affects muscle strength and is characterized by progressive muscle weakness and degeneration. While it can present with muscle weakness and wasting, it is not typically associated with polyhydramnios or decreased fetal movements.
* **Option D:** Emery-Dreifuss syndrome is a genetic disorder that affects muscle strength and is characterized by muscle weakness, wasting, and joint contractures. However, it is not typically associated with polyhydramnios or decreased fetal movements, and it is more commonly diagnosed in later childhood or adolescence.

**Clinical Pearl / High-Yield Fact**
Spinal muscular atrophy is a common genetic disorder that affects approximately 1 in 6,000 to 1 in 10,000 infants worldwide. It is essential to diagnose SMA early, as treatment with intrathecal administration of nusinersen, a spinal muscular atrophy-targeting antisense oligonucleotide, can improve motor function and survival in affected infants.

**✓ Correct Answer: A. Spinal muscular atrophy**