What is the primary sign noted in the ultrasound for Joubert syndrome?

The primary sign noted in ultrasound for Joubert syndrome is the "molar tooth sign." This term describes a characteristic appearance of the midbrain structures, specifically the malformed brainstem and cerebellar vermis, which resembles a molar tooth on imaging. The presence of this sign is crucial for the diagnosis of Joubert syndrome, as it reflects the primary neuroanatomical abnormalities associated with the condition.

This sign is specific to Joubert syndrome and serves as a key indicator for clinicians during imaging assessments. Recognizing the molar tooth sign helps differentiate Joubert syndrome from other disorders, as it reveals distinct malformations that are integral to the diagnosis.

The other choices, while they may seem related to genetic or developmental disorders, do not specifically pertain to the diagnostic features of Joubert syndrome. For instance, polydactyly, rhabdomyoma, and radial ray anomalies are associated with different conditions and do not reflect the hallmark findings in imaging that are characteristic of Joubert syndrome.