Q: Hyponatremia with a high or normal serum osmolality can occur in all of the following except?

A) ethanol ingestion
B) hyperglycemia
C) advanced renal failure
D) administration of hypertonic mannitol
E) in marathon  runners


Answer: E

Marathon and ultramarathon runners develop hyponatremia - not hypernatremia, contrary to popular belief (exercise-associated hyponatremia). It occurs due to excessive water intake combined, in many cases, with impaired water excretion due to persistent ADH secretion.

All other choices given (A, B, C and D) are associated with high or normal serum osmolality.



References:

1. Noakes TD, Sharwood K, Speedy D, et al. Three independent biological mechanisms cause exercise-associated hyponatremia: evidence from 2,135 weighed competitive athletic performances. Proc Natl Acad Sci U S A 2005; 102:18550. 

2. Almond CS, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med 2005; 352:1550. 

3. Sallis RE. Fluid balance and dysnatremias in athletes. Curr Sports Med Rep 2008; 7:S14.