Membranous nephropathy and minimal change disease can be easily ruled out as they are nephrotic syndromes. Tubulointerstitial nephritis (aka acute interstitial nephritis) can be ruled out as it causes WBC casts not RBC as seen in this question. Papillary necrosis - either has no casts or it might show WBC casts but not RBC because the problem is not in the glomeruli.

table of nomenclature on page 582 explains that proliferative just means hyper cellular glomeruli. Given the patients history of sore throat two weeks ago, now presenting with Nephritic Syndrome with RBC casts, proliferative glomerulonephritis is the only reasonable answer.

This is BS bc PSGN is like the only nephrotic/nephritic syndrome I thought I had down cold

RBC casts===> glomerulonephritis the only option there.

So actually.... Medscape says that PSGN can progressive to a proliferative glomerulonephritis mechanism and so proliferative glomerulonephritis should be considered as a differential diagnosis for PSGN.

https://emedicine.medscape.com/article/980685-overview#a5: "The presence of acute kidney injury may suggest an alternate diagnosis (eg, membranoproliferative glomerulonephritis [MPGN], Henoch-Schönlein purpura [HSP], systemic lupus erythematosus [SLE]) or a severe or worsening APSGN, such as observed in those with crescentic glomerulonephritis or rapidly progressive glomerulonephritis... Differential Diagnosis: This includes most other types of childhood glomerulonephritides. These include IgA nephropathy, membranoproliferative glomerulonephritis, hereditary nephritis, and other forms of postinfectious glomerulonephritis."

Ironically enough, this must be what they were asking, i.e. complications of PSGN, because AMBOSS (another Step resource) directly linked the above article I found before looking farther and coming across the AMBOSS section.