A= Glomerular capillary

B = Contains glomerular filtrate

C=

D= squamous epithelium

what is the black arrow?

Basement membrane thickening is one of the earliest changes in diabetic nephropathy and preceeds microalbuminuria. It is unlikely to be the cause of albumin leak and rather is a symptom of  podocyte stress in diabetes.

Widening of foot processes can be seen at the EM level.

Microabluminuria may be treated with correction of hyperglycemia and blood pressure in the absence of changes in BM thickness!