**Core Concept**
Antinuclear antibodies (ANA) are a group of autoantibodies that target the cell nucleus and are a hallmark of autoimmune diseases, particularly Systemic Lupus Erythematosus (SLE). SLE is a chronic inflammatory disease that can affect multiple organs, including the skin, joints, kidneys, and brain.

**Why the Correct Answer is Right**
In SLE, ANA are present in a high percentage of cases due to the breakdown of tolerance to nuclear antigens, leading to the production of autoantibodies against these antigens. This is thought to be due to a combination of genetic and environmental factors that disrupt the normal functioning of the immune system. The presence of ANA is a diagnostic criterion for SLE and can be detected using various assays, including indirect immunofluorescence and enzyme-linked immunosorbent assay (ELISA).

**Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it is too low, and ANA are not present in only 10% of cases of SLE.
- **Option B:** This option is incorrect because it is too high, and ANA are not present in 90% of cases of SLE.
- **Option C:** This option is incorrect because it is too vague and does not provide a specific percentage.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the presence of ANA is not exclusive to SLE and can be found in other autoimmune diseases, such as Sjögren's syndrome and scleroderma. However, a high titer of ANA in the presence of multiple clinical criteria is highly suggestive of SLE.

**Correct Answer: C. 70-80%**