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NBME 20 Answers

nbme20/Block 1/Question#37

A 1-month-old male newborn is brought to the ...

Na+: 132; K+: 3.2; Cl−: 90; HCO3−: 37

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 +2  upvote downvote
submitted by hayayah(244),

With chronic vomiting, you lose electrolytes and a lot of acid. It triggers metabolic alkalosis which is why all the serum values are low (or on the lower end of the normal range) except for bicarbonate.

ergogenic22  decreased K+ (from increased RAAS due to volume loss) and decreased Cl- (loss of HCl from the stomach), Alkalosis from loss of HCl and thus high bicarb. For this reason high to mid range K is wrong +1  
sbryant6  Wouldn't increased RAAS lead to increased Na+? The answer shows decreased Na+. +  
sbryant6  Also, remember Bulimia Nervosa is associated with hypokalemia. +  
sugaplum  so the range they gave for K is 3-6? so 3.2 is WNL then? or are we just operating on "it is on the lower end of normal in peds" +  

 +2  upvote downvote
submitted by masonkingcobra(36),

In metabolic alkalosis, potassium moves into the cells

The loss in volume through emesis triggers RAAS resulting in increased Aldosterone release and further potassium excretion


 +0  upvote downvote
submitted by ark110(0),

But what is the difference between option A and option C (132; 4.9; 90; 35)

sympathetikey  K+ shouldn't increase. It's moving into cells due to metabolic alkalosis. +  
home_run_ball  In the parietal cell of the stomach Hydrogen ions are formed from the dissociation of carbonic acid. Water is a very minor source of hydrogen ions in comparison to carbonic acid. Carbonic acid is formed from carbon dioxide and water by carbonic anhydrase. The bicarbonate ion (HCO3−) is exchanged for a chloride ion (Cl−) on the basal side of the cell and the bicarbonate diffuses into the venous blood, leading to an alkaline tide phenomenon. +  
ergogenic22  RAAS increases from volume loss, and thus more aldosterone leads to low K+ +  
sinforslide  Three reasons for hypokalemia. First, some K+ is lost in gastric fluids. Second, H+ shifts out of cells and K+ shifts into cells in metabolic alkalosis. Third, ECF volume contraction has caused increased secretion of aldosterone. +2  

 +0  upvote downvote
submitted by apop(0),

These explanations arent great. It doesnt really have much to do with RAAS activation. It has almost everything to do with a phenomenon called the alkaline tide. In chronic vomitting, you would expect Cl- to be low and K= to be low (similar to lab results in someone with bullemia). To compensate for the low Cl-, the stomach has an antiporter which exchanges Cl- for HCO3-. Therefore, Cl- will be replenished in the stomach, while HCO3- will increase in the blood, causing an alkalosis. In alkalotic states, the H+/K+ antiporter will begin to activate, shifting K+ INTO cells (hypokalemia) and increasing pH. RAAS probably plays a role in making the hypokalemia worse but the alkaline tide is more important here.

 +0  upvote downvote
submitted by iamapotato(0),

No one here bothered to actually discuss what's going on. This is a 1-month old newborn.

He vomits after feedings, 5-day history.

At 1 month old, with vomiting after feedings, it is most likely a case of PYLORIC STENOSIS due to hypertrophy of the pyloric sphincter (usually occurs 6 weeks later).

Due to him vomiting stomach contents, he will LOSE Cl-, K+, and Na+.

He will ALSO lose H+. This is why you go into the metabolic alkalosis with a resultant increase in the HCO3-.