Tuesday, August 25, 2015

#OUWB Renal question: The bad quiz question

Apparently there was a weekly quiz and one of the questions was as follows:

And I received an e-mail asking e to answer this question. Lets go through it item by item. The stem sets up a patient with diarrhea induced metabolic acidosis. This is a cause of non-anion gap metabolic acidosis due to GI loss of bicarbonate.

Choice A. This is wrong. The filtered load of bicarbonate is dependent solely on the plasma bicarbonate concentration. The lower bicarbonate concentration seen in all metabolic acidosis would cause decreased not increased filtered load.

Choice B. This is right. Ammonia is produced in the proximal tubule in response to metabolic acidosis and hypokalemia. This is why ammonium excretion is able to be up-regulated due to an acid load. Titratable acid is fixed and can't accommodate an increased acid load. The ammonia production varies depending on metabolic need and chronic diarrhea would up-regualte ammonia production so it could be converted to ammonium in the medullary collecting duct to help clear the excess acid load.

Choice C. This is right. Hydrogen secretion is increased in the distal nephron in response to the metabolic acidosis. This is needed to replace the bicarbonate lost in the stool. Every hydrogen in secreted int eh distal nephron synthesizes a new bicarbonate molecule for the body.

Choice D. This is wrong. Diarrhea causes a non-anion gap or normal anion gap metabolic acidosis.

Choice E. This is right. Hydrogen secretion in the distal nephron is stoichiometrically equivalent to producing new bicarbonate, one cannot happen without the other.

Some of the social media action around this question:

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