**Core Concept**
Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by the production of autoantibodies against nuclear and cytoplasmic components, leading to inflammation in various tissues and organs. The disease can manifest with a wide range of symptoms, including skin rashes, joint pain, and musculoskeletal symptoms.

**Why the Correct Answer is Right**
The presence of antibodies to double-stranded DNA (anti-dsDNA) is a highly specific marker for SLE. These antibodies are directed against the double-stranded DNA and are often used as a diagnostic criterion for the disease. The production of anti-dsDNA antibodies is thought to be involved in the pathogenesis of SLE, particularly in the formation of immune complexes that deposit in various tissues and cause inflammation. In this patient, the presence of a characteristic butterfly-shaped rash on the face, joint pain, and the absence of joint deformity or radiographic abnormalities are all consistent with SLE.

**Why Each Wrong Option is Incorrect**

**Option A:** Elevated anti-streptolysin O (ASO) titer is characteristic of acute rheumatic fever, not SLE. ASO titer measures the immune response to group A beta-hemolytic streptococcal infections and is used to diagnose rheumatic fever.

**Option B:** HLA-B27 genotype is associated with an increased risk of developing ankylosing spondylitis, a type of spondyloarthritis, but it is not specific for SLE. HLA-B27 is also associated with other autoimmune diseases, such as reactive arthritis and psoriatic arthritis.

**Option C:** Markedly decreased serum level of immunoglobulin G (IgG) is not characteristic of SLE. In fact, patients with SLE often have elevated levels of IgG and other immunoglobulins due to the polyclonal activation of B cells.

**Clinical Pearl / High-Yield Fact**
It is essential to note that SLE is a diagnosis of exclusion, and other conditions that can mimic SLE, such as dermatomyositis or rheumatoid arthritis, should be ruled out before making a diagnosis. Additionally, patients with SLE are at an increased risk of developing other autoimmune diseases, such as thrombocytopenia or nephritis.

**✓ Correct Answer: D. Antibodies to double-standed DNA**