NBME 20 Urine Na+: increased; Serum osmolality: decreased (NBME Answers)

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submitted by ∗hayayah(1212)

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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 which leads to normalization of extracellular fluid volume and the euvolemic hyponatremia.

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hello Why would body respond to water retention with ALDO? ALDO would increase water retention... +7

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 +37

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 +4

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. +

srmtn this is a paraneoplasic SX related to small cell Ca. so increase in ADH (SIADH) makes free water retention (increase in weight) and dilutional hipoNa in serum and hiperNa+ in urine. sodium is the main factor in osmolality so since is hipoNa+ the osm will be decreased. +

skonys I think it's as simple as SIADH -> Inc ADH secretion therefore increased free water reabsorption (Dec Serum Osmolality) and Increased Urine Na+ because there's less water (less dilute/more concentrated). Remember that ADH creates a positive Na+ and Urea gradiant in the collecting duct to facilitate their reabsorption in the medulla (thus reestablishing the gradient. Here are the criteria from https://www.ncbi.nlm.nih.gov/books/NBK507777/ The Schwartz and Bartter Clinical Criterion Serum sodium less than 135mEq/L Serum osmolality less than 275 mOsm/kg Urine sodium greater than 40 mEq/L (due to ADH-mediated free water absorption from renal collecting tubules) +1