fibromuscular dysplasia in the left renal a. causes low flow resulting in low GFR. Chronic low GFR causes tubular atrophy. (excerpt and reference below).

Tubular atrophy is a general term that describes several patterns of chronic tubular injury with thickened tubular basement membranes, and clinically manifests as chronic kidney disease with decreased glomerular filtration rate. Increased extent of tubular atrophy and accompanying interstitial fibrosis correlates with worse prognosis. Proteinuria is variable, depending on cause.

https://www.ajkd.org/article/S0272-6386(16)30033-6/fulltext

found the explanation of question id 453 in uworld helpful

Dr. Sattar says that decreased blood flow --> atrophy. Fibromuscular dysplasia of the left renal artery is essentially renal artery stenosis --> atrophy of the left kidney

ch 1.2 pathoma cellular injury kidney will undergo ATROPHY due to slow, progressive ischemia because the blood supply is cut off. examples where blood supply is cut off to the kidney 1) atherosclerosis of renal artery 2) fibromuscular dysplasia of renal artery

fibrinoid deposits happen in PAN (vasculitis), hypertensive emergency, and pre eclampsia.

i can see why this is confusing because both PAN and fibromuscular dysplasia have that string of pearls appearance