In a 27-year-old woman presenting with lethargy, one-month weight loss, petechiae and rash on the lower limbs, prolonged activated partial thromboplastin time (aPTT), proteinuria with hematuria, acute kidney injury (AKI), alopecia, and bicytopenia (anemia and thrombocytopenia), what is the most likely diagnosis and what initial work‑up and treatment should be undertaken?

Systemic Lupus Erythematosus with Class III/IV Lupus Nephritis

This 27-year-old woman has systemic lupus erythematosus (SLE) with severe lupus nephritis (LN), and she requires immediate treatment with high-dose corticosteroids combined with either cyclophosphamide or mycophenolate mofetil (MMF), along with hydroxychloroquine.

Clinical Diagnosis

The constellation of findings makes SLE with lupus nephritis the overwhelming diagnosis:

• Constitutional: Lethargy, weight loss (1 month duration)
• Dermatologic: Petechiae, rash on lower limbs, alopecia
• Hematologic: Bicytopenia (anemia + thrombocytopenia)
• Renal: Proteinuria with hematuria, acute kidney injury
• Coagulation: Prolonged aPTT (suggesting lupus anticoagulant/antiphospholipid antibodies)

This presentation fulfills multiple SLE classification criteria and indicates active, severe disease with major organ involvement.

Immediate Diagnostic Work-Up

Essential serologic tests to confirm SLE:

• ANA, anti-dsDNA antibodies
• Complement levels (C3, C4, CH50) - expect low levels
• Complete blood count with peripheral smear (to assess for hemolysis, schistocytes)
• Direct Coombs test
• Comprehensive metabolic panel including creatinine
• Urinalysis with microscopy (looking for RBC casts, dysmorphic RBCs)
• 24-hour urine protein or spot urine protein-to-creatinine ratio

For the prolonged aPTT:

• Lupus anticoagulant panel
• Anticardiolipin antibodies (IgG and IgM)
• Anti-β2-glycoprotein I antibodies (IgG and IgM)
• Mixing study to differentiate inhibitor from factor deficiency 1, 2

Urgent renal biopsy is indispensable to determine the class of lupus nephritis and guide immunosuppressive therapy 3. The biopsy should be performed as soon as coagulation parameters allow (correct thrombocytopenia to >50,000-60,000/μL if possible).

Initial Treatment Strategy

First-Line Immunosuppression

For Class III or IV lupus nephritis (most likely given AKI with proteinuria/hematuria), initiate combination therapy immediately 4, 3:

Corticosteroids (mandatory):

• Methylprednisolone 1g IV daily for 3 days (pulse therapy), followed by
• Prednisone 1 mg/kg/day orally (maximum 80 mg) as a single daily dose 4
• Continue high-dose for minimum 4 weeks, up to 16 weeks or until complete remission 4
• Taper slowly over 6 months after achieving complete remission 4

PLUS one of the following:

Mycophenolate mofetil (MMF) - preferred as first-line due to better efficacy/toxicity ratio 3:

• Standard dosing for induction therapy
• Continue for at least 3 years as maintenance 3

OR Cyclophosphamide (CYC):

• Low-dose IV cyclophosphamide regimen for standard cases 3
• Higher doses reserved for adverse clinical/histological features (extensive crescents, fibrinoid necrosis, severe interstitial infiltrate) 4
• Adjust dose for age and eGFR 5

Essential Adjunctive Therapy

Hydroxychloroquine is recommended for ALL patients with lupus nephritis 3:

• Continue indefinitely unless contraindicated
• Reduces flares and improves long-term outcomes

Supportive care:

• ACE inhibitor or ARB for proteinuria reduction and blood pressure control (target BP <130/80 mmHg)
• Anticoagulation consideration: Given prolonged aPTT suggesting antiphospholipid antibodies, if lupus anticoagulant or antiphospholipid antibodies are confirmed, consider prophylactic anticoagulation, especially with nephrotic-range proteinuria 3

Critical Pitfalls to Avoid

Do not delay treatment waiting for biopsy results if the patient has rapidly worsening renal function or life-threatening manifestations. Start corticosteroids immediately and add cyclophosphamide or MMF empirically 4.

Do not use immunosuppression if:

• Serum creatinine ≥3.5 mg/dL with small echogenic kidneys on ultrasound 5
• Active severe infection is present

Monitor for treatment response:

• If worsening occurs (rising creatinine, worsening proteinuria) during first 3 months, switch to alternative therapy or repeat kidney biopsy 4
• Manage conservatively for at least 6 months after completing initial regimen before declaring treatment failure 5

Antiphospholipid syndrome considerations:

• The prolonged aPTT may indicate lupus anticoagulant, which can cause both thrombosis and interfere with aPTT monitoring if using unfractionated heparin 1
• If antiphospholipid antibodies are confirmed, long-term anticoagulation will be needed 3, 6, 7, 8
• Distinguish from thrombotic thrombocytopenic purpura (TTP) by checking ADAMTS13 activity - though TTP would typically present with more severe thrombocytopenia and neurologic symptoms 9, 10

Infection prophylaxis during high-dose immunosuppression:

• Pneumocystis jirovecii prophylaxis (trimethoprim-sulfamethoxazole)
• Screen for latent tuberculosis, hepatitis B/C before starting therapy 11

Expected Outcomes

The goal is complete renal response: proteinuria <0.5 g/24h with normal or near-normal renal function 3. Early aggressive treatment with the combination regimen offers the best chance of preserving kidney function and preventing progression to end-stage renal disease. The presence of AKI at presentation indicates more aggressive disease requiring close monitoring and potentially more intensive therapy 4.