**Core Concept:** Allelic heterogeneity refers to the presence of multiple alleles for a particular gene in a population. In other words, it is the existence of different variants of a gene at the same locus on a chromosome. This concept is important in understanding genetic disorders and their inheritance patterns.

**Why the Correct Answer is Right:** Option D, "Huntington's Disease", exhibits allelic heterogeneity because it is caused by mutations in a single gene, the HTT gene, which contains multiple alleles. The most well-known allele is the CAG trinucleotide repeat expansion, which leads to the disease. The other alleles may have different lengths of CAG repeats, but they all result in the same clinical phenotype: Huntington's Disease.

**Why Each Wrong Option is Incorrect:**

A. "Cystic Fibrosis" is caused by mutations in the CFTR gene, which has a high degree of genetic heterogeneity. Different mutations lead to different clinical presentations, making it an example of genetic heterogeneity, not allelic heterogeneity.

B. "Sickle Cell Anemia" is also caused by mutations in a single gene (HBB), but these mutations lead to different amino acid substitutions in the hemoglobin protein. This represents genetic heterogeneity, not allelic heterogeneity.

C. "Thalassemia" is caused by mutations in different genes (α- and β-globin genes), which results in genetic heterogeneity, not allelic heterogeneity.

**Clinical Pearl / High-Yield Fact:** Allelic heterogeneity is important in understanding the genetic basis of inherited disorders and their clinical presentation. Recognizing allelic heterogeneity helps differentiate diseases based on the specific mutation and provides insights into disease management and prognosis.