Multiple System Atrophy (MSA): This condition is characterized by parkinsonism, autonomic dysfunction (such as postural hypotension), and other systemic features. Anterocollis, or forward flexion of the neck, is a distinctive feature that can be seen in MSA, making it a strong candidate for the single most likely diagnosis.

Parkinson's Disease with Autonomic Dysfunction: While less common, some patients with Parkinson's disease can develop significant autonomic dysfunction, including postural hypotension. However, anterocollis is less typical for Parkinson's disease.
Dementia with Lewy Bodies (DLB): DLB can present with parkinsonism, fluctuations in cognitive function, visual hallucinations, and autonomic dysfunction, including postural hypotension. Anterocollis might be seen but is not as characteristic as in MSA.
Progressive Supranuclear Palsy (PSP): PSP is known for causing parkinsonism and distinctive oculomotor abnormalities. Some patients may exhibit postural instability and falls, but postural hypotension is less common. Anterocollis can be a feature, particularly in the context of axial rigidity.

Pheochromocytoma: Although rare, pheochromocytoma can cause episodic hypertension but also orthostatic hypotension due to volume depletion or autonomic dysfunction. Parkinsonism is not a typical presentation, but it could be considered in the context of drug-induced parkinsonism or as part of a multiple endocrine neoplasia syndrome.
Autoimmune Autonomic Ganglionopathy: This is a rare autoimmune disorder that can cause severe autonomic dysfunction, including postural hypotension. While parkinsonism is not a primary feature, the significant autonomic dysfunction warrants consideration to avoid missing a potentially treatable condition.

Familial Dysautonomia (Riley-Day Syndrome): A rare genetic disorder that affects the development of the autonomic nervous system, leading to severe autonomic dysfunction. Parkinsonism is not a characteristic feature, but the presence of postural hypotension and other autonomic symptoms might lead to consideration in specific clinical contexts.
Neurodegeneration with Brain Iron Accumulation (NBIA): A group of rare, inherited disorders characterized by progressive neurological deterioration and iron accumulation in the brain. Some forms can present with parkinsonism and dystonia, but postural hypotension and anterocollis would be unusual as primary features.