Give the most probable diagnosis of a 1 year. old child of normal intelligence with features of hypotonia. On examination, there are tongue fasciculations and he keeps his body in a frog-like position –
Features of hypotonic and frog-like position suggest the diagnosis of Floppy infant syndrome.

Floppy infant syndrome

In FIS the baby is floppy i.e :
He cannot hold his head up, there will be marked head lag as the baby is pulled up from supine to sitting posture.
A normal infant, on ventral suspension (i.e. supporting the infant with a hand under the chest) keeps his back straight and head slightly up, arms abducted and knees partly flexed. A floppy infant droops, over the hand like an inverted U.

Typical features 

Hypotonia and decreased muscular activity
Frog-like posture or other unusual position at rest
Delay in motor milestones

Causes of Floppy infant syndrome
Central nervous system

Perinatal asphyxia, neonatal encephalopathy, kernicterus, cerebral palsy (atonic type), intracranial haemorrhage, chromosomal anomalies including Down syndrome and inborn errors of metabolism e.g., aminoacidurias, mucopolysaccharidosis, and cerebral lipidosis.


Spinal cord lesions

Anterior horn cell disease: Werdnig Hoffman spinal muscular atrophy, poliomyelitis

Peripheral nerves

Acute polyneuropathy, familial dysautonomia, congenital sensory neuropathy

Myoneural junction

Neonatal myasthenia gravis, infantile botulism, following antibiotic therapy

Muscles

Muscular dystrophies, congenital myotonic dystrophies, congenital myopathies (including central core disease and Nemaline myopathy), polymyositis, glycogen storage disease (Pompe's), and arthrogryposis multiplex congenital

Miscellaneous

Protein-energy malnutrition, rickets, Prader-Willi syndrome, malabsorption syndromes, Ehler-Danlos syndrome, cutis laxa, cretinism

Coming back to question

There are many causes of floppy infant syndrome.
Out of the four option given in the question three of these can cause FIS -
Down’s syndrome


GB. syndrome
Spinal muscular atrophy

Spinal muscular atrophy

Features of normal intelligence and tongue fasciculation's strongly suggested spinal muscular atrophy.

 Down's syndrome

Can be easily ruled out as intelligence is impaired in Down's syndrome.

GB. syndrome

Can be ruled out as it presents as an acute case with a past history of fever or viral infections.
Majority of patients recover within a few days to 2 weeks.
Guillian Barre Syndrome
Limb girdle atrophy
Down's syndrome
Spinal muscular atrophy