For a patient with lupus presenting with nephrotic syndrome symptoms, which diagnostic study is best for determining the cause of proteinuria?

A renal biopsy is the best diagnostic study for determining the cause of proteinuria in a patient with lupus presenting with nephrotic syndrome symptoms. This procedure allows for direct examination of renal tissue, which can reveal specific histopathological changes indicative of lupus nephritis or other renal pathologies. In the context of systemic lupus erythematosus (SLE), renal involvement can manifest in various ways, often requiring a biopsy to differentiate between different types of glomerulonephritis that may lead to proteinuria.

The renal biopsy provides critical information on the level of kidney involvement, the type of damage present (such as immune complex deposition), and helps guide treatment decisions. While other imaging studies like renal ultrasound or CT scans can provide some information about kidney structure, they do not allow for the assessment of the underlying causes of proteinuria at a cellular level. Therefore, they are not as definitive for this purpose. Cystoscopy is designed primarily for evaluation of the bladder and does not provide information relevant to kidney pathology.