Final answer:
The essential clinical features that establish a diagnosis of Parkinson's disease include muscle tremor, muscle rigidity, slowness of movement, and postural instability.
Step-by-step explanation:
Parkinson's disease is a degenerative disorder of the central nervous system that mainly affects the muscular system and movement. Four motor signs and symptoms are considered defining Parkinson's disease: muscle tremor (shaking), muscle rigidity, slowness of movement, and postural instability.
Tremor is the most common and obvious symptom, and it most often occurs in a limb at rest, so it disappears during sleep or when the patient moves the limb voluntarily. Difficulty walking eventually develops, and dementia is common in the advanced stages of the disease. Depression is common, as well.
The presence of Lewy bodies, abnormal protein clumps found within the affected neurons, is also a significant marker of the disease. While some cases are due to genetic mutations, the majority are thought to result from a mix of environmental and genetic factors with no single known cause.