A 35-year-old woman presents with a malar rash, arthralgias, and positive ANA with anti-dsDNA antibodies. Urinalysis shows proteinuria and RBC casts. Which class of lupus nephritis carries the worst prognosis if untreated?
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A
Class I (minimal mesangial)
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B
Class III (focal proliferative)
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C
Class IV (diffuse proliferative)
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D
Class V (membranous)