Drugs clinically administered to patients must be eliminated by the body.  One route of elimination is via the liver, in which hepatocytes endocytose drugs and transport them to different cellular areas using motor proteins that interact with tubulin proteins.The kidney also plays an important role in drug elimination.  During the development of new drugs, researchers use a common benchmark: if 30% or more of the drug dose is excreted in the urine in a form unchanged from administration, the kidney plays a significant role in elimination of that drug.  However, nephron proximal tubules contain peptidases that can metabolize some substances.  For example, a significant portion of the antibiotic drug imipenem undergoes renal metabolism prior to elimination in the urine.Nephrotoxicity, a type of kidney damage, can be caused by drugs.  For instance, the chemotherapy drug cisplatin can lead to nephrotoxicity by promoting excessive fission of mitochondria in renal tubular cells.  Patients with reduced kidney function or a transplanted kidney may experience drug-induced nephrotoxicity at drug doses that would not cause damage to patients with healthy kidney function.Table 1 shows results of a study in which the elimination of the enzyme superoxide dismutase (SOD) was examined in patients.  In the study, average total clearance of SOD was measured at different doses, along with the portion of that clearance that was performed by the kidney.  The standard error for clearance values is 0.05 L/h.Table 1  Renal and total clearance of superoxide dismutase (SOD)Brater, D. C. (2002). Measurement of renal function during drug development. British Journal of Clinical Pharmacology, 54(1), 87–95. Question 56Based on the passage, imipenem is most likely:A.smaller than pores found in glomerular capillaries.B.found only in its unchanged form in the loop of Henle.C.too large to be filtered by nephrons.D.secreted largely into the distal convoluted tubule.