**Core Concept**
Type I diabetes mellitus is an autoimmune disease characterized by the destruction of pancreatic beta cells, leading to insulin deficiency. The disease is associated with specific genetic markers, particularly HLA (Human Leukocyte Antigen) genes. The risk of developing type I diabetes is influenced by the presence of certain HLA haplotypes.

**Why the Correct Answer is Right**
The correct answer is linked to the HLA complex, specifically the HLA-DR and HLA-DQ genes, which are crucial in the immune system's recognition of self and non-self antigens. The presence of the HLA-DR3 and HLA-DR4 haplotypes, along with HLA-DQ2 and HLA-DQ8, increases the risk of developing type I diabetes. These haplotypes are associated with an increased expression of autoantibodies and an impaired function of regulatory T cells, contributing to the disease's pathogenesis.

**Why Each Wrong Option is Incorrect**
**Option A:** HLA-A1 and HLA-A2 haplotypes are not specifically associated with an increased risk of type I diabetes.
**Option B:** HLA-B27 is more commonly linked to ankylosing spondylitis and other spondyloarthropathies.
**Option C:** HLA-Cw6 is associated with psoriasis, not type I diabetes.

**Clinical Pearl / High-Yield Fact**
Family members of individuals with type I diabetes, particularly first-degree relatives, have a 5-15% risk of developing the disease, making genetic screening and monitoring essential for early detection and prevention.

**Correct Answer: D. HLA-DR3 and HLA-DR4 haplotypes.**