Parkinson's Disease: The patient's symptoms of a worsening tremor over six months, shaky writing, and the presence of tremor at rest are classic for Parkinson's disease. The disease often presents unilaterally and progresses over time.

Essential Tremor: Although essential tremor typically presents with action tremors (e.g., during writing), some patients may exhibit rest tremors. The condition is more common than Parkinson's disease and can cause significant disability.
Dystonic Tremor: This condition can cause tremors, often accompanied by dystonic posturing. The tremor can be present at rest or with action.

Wilson's Disease: A genetic disorder that leads to copper accumulation in the brain and liver. It can cause tremors, among other neurological symptoms, and is potentially treatable if diagnosed early. Missing this diagnosis could lead to severe neurological deterioration and death.
Thyroid Dysfunction: Both hyperthyroidism and hypothyroidism can cause tremors. Although less likely to cause a unilateral rest tremor, thyroid disorders are common and easily treatable, making them important not to miss.
Medication-Induced Tremor: Certain medications can cause tremors as a side effect. Identifying and potentially changing the offending medication could resolve the tremor.

Multiple System Atrophy (MSA): A rare neurodegenerative disorder that can present with tremors, among other symptoms like autonomic dysfunction and ataxia. It is less common than Parkinson's disease but can have a similar presentation.
Progressive Supranuclear Palsy (PSP): Another rare neurodegenerative disease that can cause tremors, although it more commonly presents with difficulties in moving the eyes and balance problems.
Cerebellar Disorders: Conditions affecting the cerebellum, such as cerebellitis or a cerebellar stroke, can cause tremors, typically intention tremors, but are less likely to cause a unilateral rest tremor.