The most likely diagnosis is interstitial nephritis caused by mesalamine therapy. Interstitial nephritis is characterized by an inflammatory infiltrate into the kidney interstitium that can lead to tubular dysfunction and kidney failure. Interstitial nephritis may be associated with autoimmune diseases and infections but is most commonly caused by drugs. Mesalamine-induced interstitial nephritis is a well-described complication that can be either acute or chronic and may occur months to years after exposure, even in patients who have safely tolerated the medication in the past. Sterile pyuria and leukocyte casts are hallmarks of interstitial nephritis, which can present acutely or may progress indolently and present as chronic kidney disease of unclear duration. Mild subnephrotic proteinuria also can be seen with interstitial nephritis.

Kidney disease in systemic lupus erythematosus is relatively common and may have a variable presentation, often with significant hematuria, proteinuria, and cellular casts. However, lupus nephritis is unlikely in this patient because of the absence of extrarenal lupus manifestations, his male gender, and lack of significant proteinuria or hematuria.

Membranous glomerulopathy primarily affects the glomerulus and is therefore associated with heavy proteinuria and the nephrotic syndrome, making this an unlikely diagnosis for this patient.

Rapidly progressive glomerulonephritis is associated with hematuria and erythrocyte casts and variable proteinuria, usually with other clinical manifestations such as hypertension. In some cases, other systemic symptoms or clinical findings associated with an underlying etiology may be present, such as pulmonary hemorrhage or upper and lower respiratory tract involvement. None of these findings is present in this patient, making this diagnosis unlikely.