What ultrasound finding is indicative of Joubert syndrome?

The ultrasound finding that is indicative of Joubert syndrome is the "molar tooth sign." This sign refers to a characteristic appearance of the brain seen on imaging studies. Specifically, in Joubert syndrome, the molar tooth sign consists of a malformation of the brainstem and cerebellum, where the superior medullary velum and the deep cerebellar structures appear to resemble the shape of a molar tooth. This distinctive feature is highly suggestive of Joubert syndrome and is used as a key diagnostic criterion.

Other options presented, such as rhabdomyoma, interrupted aortic arch, and telephone receiver femur, are associated with different conditions. Rhabdomyomas are typically linked to tuberous sclerosis, interrupted aortic arch is a heart defect that can occur in various syndromes, and the telephone receiver femur is often associated with diseases such as achondroplasia. These findings do not pertain to Joubert syndrome, establishing the molar tooth sign as the definitive answer for identifying this specific genetic condition.