A 28-year-old woman from Delhi presents with a 6-month history of polyarthritis affecting her hands, wrists, and knees, along with persistent fatigue and a malar rash. She reports photosensitivity and has had two episodes of oral ulcers. On examination, she has a butterfly rash across her cheeks and nasal bridge. Laboratory investigations reveal: ANA positive (1:640, homogeneous pattern), anti-dsDNA antibodies positive (180 IU/mL), anti-Smith antibodies positive, complement levels low (C3 18 mg/dL, C4 12 mg/dL), and urinalysis shows 2+ proteinuria with RBC casts. What is the most likely diagnosis?

Explanation

## Diagnosis: Systemic Lupus Erythematosus with Lupus Nephritis ### Clinical Presentation This patient meets the 2019 EULAR/ACR classification criteria for SLE with multiple organ involvement: - **Mucocutaneous:** malar rash, photosensitivity, oral ulcers - **Musculoskeletal:** polyarthritis (hands, wrists, knees) - **Constitutional:** fatigue - **Renal:** proteinuria with RBC casts (WHO Class III/IV lupus nephritis) ### Serological Findings | Finding | Significance | |---------|-------------| | ANA positive (homogeneous) | Sensitive for SLE (95–98%) | | Anti-dsDNA positive | Highly specific for SLE (70–90%) | | Anti-Smith positive | Highly specific for SLE (25–30% of patients) | | Low C3/C4 | Indicates active disease; associated with lupus nephritis | **Key Point:** Anti-dsDNA and anti-Smith antibodies together are virtually diagnostic of SLE; anti-Smith is 99% specific. ### Renal Involvement **High-Yield:** Proteinuria with RBC casts indicates active lupus nephritis. The presence of: - Active urinary sediment (RBC casts) - Proteinuria (≥0.5 g/day) - Low complement levels ...confirms proliferative glomerulonephritis (WHO Class III or IV), which requires urgent immunosuppressive therapy. **Clinical Pearl:** Lupus nephritis occurs in 40–50% of SLE patients and is a major cause of morbidity and mortality. Early recognition and treatment prevent progression to ESRD. ### Why This Is SLE, Not Other Diagnoses - **Not MCTD:** MCTD typically has anti-RNP antibodies (negative here), less prominent renal disease, and lower anti-dsDNA positivity. - **Not RA:** RA does not produce ANA, anti-dsDNA, or anti-Smith; lacks photosensitivity and malar rash. - **Not drug-induced lupus:** Drug-induced lupus is usually anti-histone positive, anti-dsDNA and anti-Smith negative, and lacks renal involvement. **Mnemonic for SLE classification criteria (EULAR/ACR 2019):** **SOAP BRAIN** (mnemonic for 11 criteria: Serology [ANA, anti-dsDNA, anti-Smith], Oral ulcers, Arthritis, Photosensitivity, Blood cytopenias, Renal, Acute phase reactants, Immunology [low complement], Neurologic).