Diagnose This (May 3, 2021)

This image is from a biopsy of a young female who presented with proteinuria during pregnancy. Routine immunofluorescence was negative. What is this stain and what is your diagnosis?

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The stain depicted is Serum Amyloid Protein (SAP) immunofluorescence. SAP has been recently described as a marker that colocalizes with IgG in cases of membranous-like glomerulopathy with masked IgG kappa deposits. This entity typically presents in young females, many of which have positive autoimmune serologies but no well-defined autoimmune disease. Although the deposits are monotypic (usually IgG1 kappa), the majority of patients do not have a detectable monoclonal paraprotein, and so far no cases associated with hematologic malignancy have been described.

https://doi.org/10.1016/j.kint.2019.10.026

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5678740/