**Core Concept:** Beta thalassemia is an inherited blood disorder characterized by reduced or absent synthesis of beta-globin chains. The most common gene mutation in this condition is related to the HBB gene, which encodes for the beta-globin chain.

**Why the Correct Answer is Right:** Beta thalassemia is caused by mutations in the HBB gene on chromosome 11, which encodes for the beta-globin chain. The most common mutation is a single nucleotide change (C to T) at nucleotide position +87 (C87T), resulting in the amino acid substitution of arginine for glutamine at position 32 (R32Q) in the beta-globin chain. This mutation leads to reduced synthesis of the beta-globin chain, causing a deficiency in hemoglobin A2.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the C87T mutation is the most common gene mutation in beta thalassemia, not a different mutation.
B. This option is incorrect because the C87T mutation is the most common gene mutation in beta thalassemia, not a different mutation.
C. This option is incorrect because the C87T mutation is the most common gene mutation in beta thalassemia, not a different mutation.
D. This option is incorrect because the C87T mutation is the most common gene mutation in beta thalassemia, not a different mutation.

**Clinical Pearl:** Understanding the genetics and molecular basis of beta-thalassemia mutations is essential for accurate diagnosis, counseling, and management of patients affected by these disorders. Knowledge of the most common mutations, such as C87T, allows healthcare providers to make informed decisions regarding blood transfusions, chelation therapy, and bone marrow transplantation for affected individuals.