**Core Concept:** Sickle cell disease is a genetic disorder caused by mutations in the HBB gene, which codes for the beta-globin subunit of the hemoglobin molecule. The most common type mutation is a single nucleotide change (C to T) at position -108 of the HBB promoter region, leading to altered transcription of the gene and reduced beta-globin production.

**Why the Correct Answer is Right:** The correct answer, D (C to T mutation at -108), refers to a specific type of mutation found in the HBB gene that results in sickle cell disease. This type of mutation is a single nucleotide change, which alters the transcription of the gene and leads to reduced beta-globin production.

**Why Each Wrong Option is Incorrect:**

A. Option A (C to T mutation at +4) is incorrect because this mutation occurs at a different position in the HBB gene, leading to a different type of mutation and not causing sickle cell disease.

B. Option B (A to G mutation at +3) is incorrect because this mutation also occurs at a different position in the HBB gene, leading to a different type of mutation and not causing sickle cell disease.

C. Option C (C to T mutation at -108) is incorrect because it is the correct answer to the question and has already been explained as the cause of sickle cell disease.

**Clinical Pearl:** The correct answer (C to T mutation at -108) is crucial to understanding the molecular basis of sickle cell disease and its clinical manifestations, including hemolytic anemia, vaso-occlusive crises, and other complications such as acute chest syndrome, stroke, and osteoporosis. It is essential for medical students and healthcare professionals to know the types of mutations leading to sickle cell disease to provide appropriate genetic counseling, diagnosis, and management of patients with this disorder.