A 63-year-old man has had chronic arthritis for the past 15 years. Physical examination shows ulnar deviation with bony ankylosis producing swan neck deformities of the fingers. Laboratory studies show 4.2 g of protein in a 24-hour urine collection, serum creatinine of 3.1 g/ dL, and urea nitrogen of 3 g/dL. Level of C-reactive protein is markedly elevated. A rectal biopsy is performed, which shows deposition of amorphous pink material with H&E staining in the mucosa. The material stains positive with Congo red. Which of the following proteins is the most likely precursor to this material in the mucosa?

In chronic inflammatory conditions such as rheumatoid arthritis, the serum amyloid-associated (SAA) precursor protein forms the major amyloid fibril protein AA. SAA is an acute phase reactant that increases with inflammatory conditions. C-reactive protein also is an acute phase reactant whose level is increased in inflammatory conditions; unlike SAA, however, this does not form amyloid. Amyloid is deposited in interstitial locations. All amyloid shows characteristic "apple- green" birefringence under polarized light microscopy after Congo red staining-anything else would not be amyloid. Amyloid derived from b2 -microglobulin occurs with hemodialysis-associated amyloidosis. Amyloid derived from light chains in association with multiple myeloma has AL fibrils. Cardiac and heredofamilial forms of amyloidosis have fibrils derived from prealbumin transthyretin (TTR).