A 40-year-old man is brought to the doctor by his family for the rapid intellectual decline. Examination shows fast semi-purposive movements in hands. His father and grandfather had a similar illness. What is the diagnosis?
**Core Concept**
The question describes a patient with rapid intellectual decline and fast semi-purposive movements in hands, with a family history of similar illness. This presentation is suggestive of a neurodegenerative disorder with a strong genetic component.
**Why the Correct Answer is Right**
The patient's symptoms, including rapid intellectual decline and fast semi-purposive movements (also known as myoclonus), are characteristic of Huntington's disease (HD). HD is an autosomal dominant disorder caused by an expansion of CAG repeats in the Huntingtin gene, leading to the degeneration of neurons in the basal ganglia and cerebral cortex. This results in the characteristic motor symptoms, cognitive decline, and psychiatric features of the disease. The family history of similar illness further supports this diagnosis.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is unlikely to be correct, as Alzheimer's disease typically presents with a gradual decline in cognitive function without the characteristic motor symptoms of myoclonus.
**Option B:** Parkinson's disease is primarily characterized by tremors, rigidity, bradykinesia, and postural instability, which are not consistent with the patient's symptoms.
**Option C:** Frontotemporal dementia (FTD) is a group of disorders that primarily affect the frontal and temporal lobes, leading to changes in personality, behavior, and language. While FTD can present with cognitive decline, it is not typically associated with the characteristic motor symptoms of myoclonus.
**Clinical Pearl / High-Yield Fact**
Huntington's disease is an autosomal dominant disorder, meaning that a single copy of the mutated gene is sufficient to cause the disease. This is in contrast to most other neurodegenerative disorders, which are typically autosomal recessive or sporadic.
**Correct Answer:** C. Huntington's disease