Case report of Parkinson's disease in isolated congenital anosmia with absent olfactory bulbs.

Olfactory dysfunction is an early, prodromal feature of Parkinson's disease (PD), often preceding motor symptoms by years. This has fueled the hypothesis that PD pathology may originate in the olfactory bulb (OB), where early Lewy-type α-synucleinopathy is frequently observed and from which pathology could propagate to neural systems. In this context, Arshamian et al. (2022) hypothesized that isolated congenital anosmia (ICA), a rare lifelong absence of smell typically associated with bilateral OB aplasia, might confer immunity to PD. They argued that identifying a single individual with both ICA and PD would constitute a "black swan", falsifying the claim that an intact OB is necessary for PD initiation. We describe such a case: an individual with lifelong anosmia, MRI-based evidence of bilateral OB aplasia, and a clinically established diagnosis of PD. This counterexample challenges strict OB-necessity models and suggests PD can arise despite apparent congenital absence of an intact OB.