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NBME 20 Answers

nbme20/Block 1/Question#19

A 60-year-old man comes to the physician because of ...

Urine Na+: increased; Serum osmolality: decreased

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submitted by hayayah(445),

This patient has small cell carcinoma. This type of cancer is associated with paraneoplastic syndromes such as: Cushing Syndrome, SIADH, or antibodies against Ca2+ channels (Lambert-Eaton) or neurons. Amplification of myc oncogenes is also common.

SIADH (Syndrome of inappropriate antidiuretic hormone secretion) is characterized by:

  • Excessive free water retention
  • Euvolemic hyponatremia with continued urinary Na+ excretion
  • Urine osmolality > serum osmolality

Body responds to water retention with aldosterone and ANP and BNP. That is what causes the increased urinary Na+ secretion Žwhich leads to normalization of extracellular fluid volume Žand the euvolemic hyponatremia.

hello  Why would body respond to water retention with ALDO? ALDO would increase water retention... +2  
nala_ula  @hello, the body's response is to decrease Aldosterone since there is increased volume retention and subsequently increased blood pressure. This concept confused me a lot, but I ended up just viewing it as separate responses. First, the increased volume retention leads to increase ANP and BNP secretion that lead to decreased Na+ reabsorption in the tubules (page 294 in FA 2019) and second, this increased volume basically leads to increased pressure so lets also decrease aldosterone so there is no Na+ retention (since water comes with it)... I thought it was counterintuitive to secrete so much Na+ since you're already having decreased serum osmolality (decreased Na+ concentration) because of the water retention, but I'm guessing that this is just another way our body's well intentions end up making us worse XD +6  
compasses  see page 344 FA2019 for SIADH. +  
dickass  author pasted text straight from FA but the arrows didn't copy over, inverting the original meaning +  
medninja  The idea of increasing urine Na is getting rid of water, thats why this mechanism end increasing urine Na secretion even when there are very low serum Na levels. +  




 +0  upvote downvote
submitted by dickass(16),

SIADH: + Excessive free water retention + Euvolemic hyponatremia with continued urinary Na excretion + Urine osmolality > serum osmolality

Body responds to water retention with DECREASED aldosterone and INCREASED ANP and BNP

Water retention => Less aldosterone => Less ENAC channels => Less sodium reuptake => Loss of sodium in urine