**Core Concept**
Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by the production of autoantibodies against various nuclear antigens, leading to inflammation and damage in multiple organs, including the kidneys. The renal involvement in SLE is known as lupus nephritis, which can present with a range of histological patterns. Understanding the most common histological type of nephritis seen in SLE is crucial for diagnosis and management.

**Why the Correct Answer is Right**
Lupus nephritis is primarily classified into six classes based on the renal biopsy findings. The most common type is Class III, also known as focal lupus nephritis. Class III lupus nephritis is characterized by the presence of focal segmental glomerulosclerosis (FSGS) and/or proliferative lesions in less than 50% of the glomeruli. This type of nephritis is associated with the deposition of immune complexes, including anti-double-stranded DNA (anti-dsDNA) antibodies, which are highly specific for SLE.

**Why Each Wrong Option is Incorrect**
* **Option A:** Class I lupus nephritis is characterized by minimal mesangial lupus nephritis, which is not the most common type.
* **Option B:** Class IV lupus nephritis is characterized by diffuse lupus nephritis, which is more severe but less common than Class III.
* **Option C:** Class II lupus nephritis is characterized by membranous lupus nephritis, which is less common than Class III.

**Clinical Pearl / High-Yield Fact**
The presence of hematuria and proteinuria in a patient with SLE should prompt a renal biopsy to confirm the diagnosis of lupus nephritis. The biopsy findings will guide the treatment, which may include immunosuppressive therapy and corticosteroids.

**Correct Answer:** C. Class III lupus nephritis.