An infant presents with hypotonia and hyporeflexia. During his intrauterine period, there was polyhydramnios and decreased fetal movements. Most probable diagnosis is –
Out of the given options only spinal muscular atrophy is able to cause hypotonia and hyporeflexia during infancy. Rest of them either present late in childhood or do not give the symptoms.
Polyhydramnios and decreased fetal movements can occur in any neuromuscular disorder which present during intrauterine life.
Lets see each option one by one.


Spinal muscular atrophy (SMA)

SMA is a degenerative disease of motor neurons that begin in fetal life and continue to progress in infancy and childhood.
The cardinal features are:
Hypotonia
Generalized weakness
Absent tendon stretch reflexes
Involvement of tongue face and jaw muscle but sparing of extraocular muscle and sphincters.

Congenital myotonia

Congenital myotonia or myotonia congenita is a chloride channel defect of skeletal muscle.
There are weakness and generalized hypertrophy of muscles.
Its ruled out as


Presents in early childhood (2-3 years age)
Deep tendon reflexes are preserved.

Congenital myasthenia

It's a disease of neuromuscular junction similar to myasthenia gravis except that its not autoimmune but rather due to genetic mutations.
It presents in infancy and childhood with symptom similar to myasthenia gravis.
It can be ruled out as-


It does not present in the intrauterine life (so does not cause polyhydramnios).
DTRs are preserved

Muscular dystrophies

Muscular dystrophy refers to a group of hereditary progressive diseases.
It includes
Duchene's muscular dystrophy        
Limb-girdle      
Congenital myopathies
Becker  
Myotonic dystrophic
All of these except congenital myopathies are easily ruled out as they present in childhood age. some may rarely present in infancy but never in the intrauterine life.
Congenital myopathies are the most difficult one to rule out.


They present with hypotonia in infancy.
It may also present in intrauterine life.
The only reason I'm with 'Spinal muscular atrophy' is that congenital myopathy is a very rare disease.
Spinal muscular atrophy
Congenital myasthenia
Congenital myotonia
Muscular dystrophy