**Question:** Alzheimer's disease - associated chromosome is -

A. 14
B. 21
C. 1
D. 19

**Core Concept:** Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by memory loss, cognitive decline, and behavioral abnormalities. The disease is named after Alois Alzheimer, who first described its pathological features in 1906. Alzheimer's disease has a genetic basis and is influenced by both environmental and genetic factors.

**Why the Correct Answer is Right:** Alzheimer's disease is primarily caused by mutations in the amyloid-beta precursor protein (APP) gene located on chromosome 21 (chr 21). These mutations result in the overproduction of amyloid-beta peptides that form toxic aggregates, leading to neuronal dysfunction and death, which is the primary pathological feature of Alzheimer's disease and the basis for the correct answer B (chr 21).

**Why Each Wrong Option is Incorrect:**

A. Chromosome 14 is not directly associated with Alzheimer's disease. While certain genes on chromosome 14 may be linked to increased risk, the primary genetic factor lies in the amyloid-beta precursor protein (APP) gene on chromosome 21.

C. Chromosome 1 does not have a direct link to Alzheimer's disease. Although certain genes on chromosome 1 may be associated with AD risk, the primary genetic factor is the amyloid-beta precursor protein (APP) gene on chromosome 21.

D. Chromosome 19 is not directly associated with Alzheimer's disease. Although some genes on chromosome 19 may be linked to increased risk, the primary genetic factor is the amyloid-beta precursor protein (APP) gene on chromosome 21.

**Clinical Pearl:** Alzheimer's disease is a complex disorder involving multiple genes and environmental factors, making it challenging to pinpoint a single, direct association with a specific chromosome. However, the amyloid-beta precursor protein (APP) gene on chromosome 21 plays a critical role in the pathogenesis of Alzheimer's disease. Understanding these genetic links can aid in identifying high-risk individuals and developing targeted therapies for the disease.