Tissue is the issue: when a second biopsy reveals the true diagnosis.

We describe the case of a woman with minimal glomerular changes on initial kidney biopsy. On long-term follow-up, the patient developed nephrotic proteinuria and a second kidney biopsy was performed, which revealed focal segmental glomerulosclerosis (FSGS). Findings from electron microscopy (EM) examination suggested a genetic form of FSGS. Next-generation sequencing showed heterozygosity for a mutation in . Collagen IV nephropathies can be linked to late-onset FSGS. By establishing a genetic cause of FSGS, immunosuppressive treatment can be avoided. This case emphasizes the importance of re-biopsy in cases of a non-explained rise in proteinuria. EM can be helpful in differentiating between primary and secondary FSGS and informing treatment strategies. In cases of adult-onset FSGS that cannot be categorized by clinical-pathological assessment, genetic testing should be considered.