**Question:** The following diseases are caused by point mutation EXCEPT

A. Cystic Fibrosis
B. Huntington's Disease
C. Huntington's Disease
D. Alzheimer's Disease

**Correct Answer:** D. Alzheimer's Disease

**Core Concept:**
Point mutations are changes in a single nucleotide base pair in the DNA sequence, leading to an amino acid substitution in the encoded protein. This can result in a loss or gain of function, leading to various genetic disorders.

**Why the Correct Answer is Right:**
Alzheimer's Disease is a progressive neurodegenerative disorder characterized by memory loss, cognitive decline, and behavioral changes. It is not caused by a single point mutation but rather a complex interplay of genetic and environmental factors. Although genetic factors contribute to the disease, it is not primarily caused by a single point mutation, making Alzheimer's Disease an example of a multifactorial disease.

**Why Each Wrong Option is Incorrect:**

A. Cystic Fibrosis: This disease is caused by a point mutation in the CFTR gene, leading to impaired cystic fibrosis transmembrane conductance regulator protein function and causing problems with sweat gland and lung function.

B. Huntington's Disease: This disease is caused by a point mutation in the HTT gene, leading to an expanded CAG trinucleotide repeat causing an abnormal huntingtin protein that forms aggregates, leading to neuronal dysfunction and death.

C. Huntington's Disease: As explained above, Huntington's Disease is caused by a point mutation (expanded CAG trinucleotide repeat) and not a single nucleotide change.

**Clinical Pearl:**
Understanding the underlying mechanisms of genetic diseases, like point mutations and polygenic disorders, is essential for diagnosing and counseling patients and their families. While the knowledge of genetic mutations contributes to our understanding of disease pathogenesis and development, it is crucial to recognize that not all genetic diseases are caused by single point mutations. In Alzheimer's Disease, the complex interplay of genetic and environmental factors contribute to disease progression, making it an example of a polygenic disease rather than a single mutation disease.