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MCAT Question A Day - 6/9/13 - Answer!

Membrane carrier molecules in the cells of the proximal convoluted tubules are capable of reabsorbing normal amounts of glucose that have been filtered out of the blood at the renal corpuscle. What happens when hyperglycemic blood has excess glucose filtered at the renal corpuscle?

A.  Membrane carrier molecules at the proximal convoluted tubules will increase their maximum transport rate.
B.  Extra carrier molecules will be synthesized in the cells of the proximal convoluted tubules.
C.  The excess glucose will continue in the filtrate and be excreted in the urine.
D.  Hormones will stimulate a second round of glucose reabsorption at the distal convoluted tubules.

The correct answer is (C). If an individual has high blood glucose levels, extra glucose will be filtered through Bowman’s capsule into the lumen of the nephron. However, since there are still only the normal number of glucose carrier molecules in the cells of the proximal convoluted tubules, only “normal” levels of glucose can be reabsorbed. The excess glucose that could not be reabsorbed stays in the filtrate through the entire nephron and ends up in the urine. This contributes to why individuals with diabetes mellitus have glucose in their urine.