Joubert's syndrome hallmark is best described by which finding?

• A. Molar tooth sign on MRI
• B. Autosomal dominant inheritance
• C. Onset in adulthood
• D. Hypotonia is absent

Answer: (A)

The hallmark finding is the molar tooth sign on MRI. This distinctive axial pattern results from cerebellar vermis hypoplasia with elongation and crowding of the superior cerebellar peduncles, plus a deep interpeduncular fossa, giving a shape that resembles a molar tooth. That imaging feature is highly characteristic of Joubert syndrome and helps confirm the diagnosis when seen alongside the typical clinical signs.

Clinically, the syndrome often presents in infancy with hypotonia, later developing ataxia, abnormal eye movements, and breathing abnormalities, which fits with the cerebellar and brainstem involvement suggested by the MRI sign.

In contrast, autosomal dominant inheritance is not typical for this condition (it is usually autosomal recessive), onset in adulthood is not characteristic (presentation is in infancy), and hypotonia is not absent (hypotonia is a common early finding).