A pathologist examines a renal biopsy from a 45-year-old man with nephrotic syndrome and requests a Congo red stain to confirm the nature of an amorphous acidophilic extracellular hyaline substance localized within the mesangial matrix of the glomeruli. A positive test confirms the presence of ?

(A) α -antitrypsin
(B)  amyloid
(C)  copper
(D)  glycogen. 

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