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NBME 20 Answers

nbme20/Block 1/Question#44

A 35-year-old woman with newly diagnosed ...

MR angiography of the renal arteries

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submitted by hayayah(330),

Renovascular disease is the most common cause of 2° HTN in adults. Can be d/t ischemia from renal stenosis or microvascular disease. Can hear renal bruits lateral to umbilicus.

Main causes of renal artery stenosis:

  • Atherosclerotic plaques—proximal 1/3rd of renal artery, usually in older males, smokers.

  • Fibromuscular dysplasia—distal 2/3rd of renal artery or segmental branches, usually young or middle-aged females.

Lab values based off:

  1. Stenosis decreases blood flow to glomerulus.
  2. Juxtaglomerular apparatus (JGA) responds by secreting renin, which converts angiotensinogen to angiotensin I.
  3. Angiotensin I is converted to angiotensin II (ATII) by angiotensin converting enzyme (ACE --in lungs)
  4. ATII raises blood pressure by (1) contracting arteriolar smooth muscle, increasing total peripheral resistance and (2) promoting adrenal release of aldosterone, which increases reabsorption of sodium (where Na+ goes H2O will follow) in the distal convoluted tubule (expanding plasma volume). Can lead to hypokalemia (seen in the labs for this question)
  5. Leads to HTN with increased plasma renin and unilateral atrophy (due to low blood flow) of the affected kidney; neither feature is seen in primary hypertension
uslme123  So both causes would result in increased aldo and MR is the only way to differentiate the two? +1  
hello  @USMLE123 I think both are causes of renal artery stenosis and that could be seen via MR angiography. It is asking what could help DIAGNOSE this patient -- and her most likely cause of the findings is fibromuscular dysplasia. So, yes, MR angiography would look different for the 2 different etiologies and thus could can be used to differentiate the two from one another. However, epidemiologically, we are looking to diagnose her with the suspected most probable cause. +1  
yotsubato  @USLME123 I think measuring Aldosterone is an incorrect answer because you already know its increased due to low K. Knowing she has high Aldosterone wouldnt provide you evidence for a final diagnosis. +2