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Difficulty: HardEndocrine and Metabolic Mechanisms

A 46-year-old man comes to the clinic for a follow-up visit regarding poorly controlled hypertension. Over the past 4 months, his blood pressure has consistently been in the 160s/100s mmHg despite adherence to maximum doses of amlodipine and lisinopril. He reports new-onset generalized muscle weakness, fatigue, and frequent urination. His medical history is significant for chronic dyspepsia, for which he recently began consuming large quantities of a European herbal black candy. He does not use tobacco or illicit drugs. Blood pressure is 168/98168/98 mmHg, and pulse is 78/min. Physical examination reveals no edema, normal secondary sexual characteristics, and no striae or easy bruising.

Laboratory studies show:
Serum sodium: 146 mEq/L
Serum potassium: 2.5 mEq/L
Serum chloride: 101 mEq/L
Serum bicarbonate: 32 mEq/L
Plasma renin activity: 0.1 ng/mL/hr (normal: 0.6-4.3 ng/mL/hr)
Plasma aldosterone: 1.2 ng/dL (normal: 3.0-30.0 ng/dL)
Morning serum cortisol: 16 μ\mu g/dL (normal: 5-23 μ\mu g/dL)

Which of the following best describes the specific molecular mechanism responsible for this patient's clinical and laboratory presentation?

  1. Inhibition of the intracellular conversion of cortisol to its inactive metabolite in the distal nephronAnswer
  2. B
    Constitutive activation of the amiloride-sensitive epithelial sodium channel in the collecting duct
  3. C
    Decreased activity of a cytochrome P450 enzyme resulting in the accumulation of 11-deoxycorticosterone
  4. D
    Ectopic paraneoplastic production of a peptide hormone that directly stimulates the zona fasciculata
  5. E
    Pharmacologic induction of hepatic enzymes accelerating the clearance of antihypertensive medications

Answer

Inhibition of the intracellular conversion of cortisol to its inactive metabolite in the distal nephron
The correct answer describes the molecular mechanism of acquired apparent mineralocorticoid excess (AME) secondary to licorice ingestion. The patient's presentation of severe hypertension, hypokalemia, and metabolic alkalosis strongly suggests a state of mineralocorticoid excess. However, both plasma renin activity and aldosterone levels are markedly suppressed, eliminating primary and secondary hyperaldosteronism. The crucial historical clue is the patient's consumption of a 'European herbal black candy' for dyspepsia, which points to authentic black licorice. Black licorice contains glycyrrhizic acid, which is hydrolyzed in the gut to glycyrrhetinic acid. In the principal cells of the renal collecting duct, glycyrrhetinic acid competitively inhibits the enzyme 11 β\beta-hydroxysteroid dehydrogenase type 2 (11 β\beta-HSD2). Normally, 11 β\beta-HSD2 rapidly converts cortisol into its inactive metabolite, cortisone. This protects the mineralocorticoid receptor (MR) from cortisol, which circulates at concentrations 100 to 1,000 times higher than aldosterone but has equal affinity for the MR. When 11 β\beta-HSD2 is inhibited, local intracellular cortisol levels rise and inappropriately activate the MR, leading to enhanced sodium reabsorption, potassium excretion, and volume expansion with subsequent suppression of the renin-angiotensin-aldosterone system.

Step-by-Step Solution

1
Identify the primary clinical syndrome.
The patient has severe, treatment-resistant hypertension, profound hypokalemia, and metabolic alkalosis, indicating a state of mineralocorticoid excess.
Recognizing the triad of hypertension, hypokalemia, and alkalosis is the first step in localizing the pathology to the adrenal-renal axis.
2
Evaluate the renin-angiotensin-aldosterone system (RAAS) profile.
Both plasma renin activity and plasma aldosterone are markedly suppressed.
This rules out primary hyperaldosteronism (Conn syndrome - high aldosterone, low renin) and secondary hyperaldosteronism (high renin, high aldosterone).
3
Formulate a differential diagnosis for low-renin, low-aldosterone hypertension.
The differential includes exogenous mineralocorticoid use, Liddle syndrome, congenital adrenal hyperplasia (11 β\beta- or 17 α\alpha-hydroxylase deficiency), Cushing syndrome, and apparent mineralocorticoid excess (AME).
A structured differential prevents premature closure on genetic causes and forces consideration of acquired etiologies.
4
Correlate the medical history with the differential diagnosis.
The recent consumption of 'European herbal black candy' for dyspepsia suggests ingestion of authentic black licorice, which contains glycyrrhizic acid.
Identifying the trigger differentiates an acquired condition from congenital or genetic causes like Liddle syndrome or CAH.
5
Determine the precise molecular mechanism of the offending agent.
Glycyrrhetinic acid (the active metabolite of licorice) competitively inhibits 11 β\beta-hydroxysteroid dehydrogenase type 2 (11 β\beta-HSD2) in the renal collecting duct, preventing the conversion of cortisol to inactive cortisone and allowing cortisol to inappropriately activate the mineralocorticoid receptor.
This establishes the final mechanistic link required to answer the question correctly.

Key Concept

Licorice-induced apparent mineralocorticoid excess (AME) is caused by the inhibition of 11 β\beta-hydroxysteroid dehydrogenase type 2 (11 β\beta-HSD2) in the renal collecting duct, leading to inappropriate activation of the mineralocorticoid receptor by circulating cortisol.
Estimated Time:1m 30s
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Topics

Endocrine and Metabolic Mechanisms

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