USMLE PRACTICE QUESTION #106

TOPIC: Endocrine Physiology & Pathology

Dec 12, 2024

In USMLE practice question #106, we’ll challenge our mastery of the endocrine system (endocrine physiology and pathology).

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A 43-year-old woman presents to her primary care physician with complaints of fatigue, unintentional weight loss, and increased skin pigmentation. She reports feeling lightheaded, especially when moving from sitting to standing. Physical examination reveals hyperpigmented patches on her gums and knuckles. Laboratory results show a serum sodium level of 130 mEq/L, serum potassium level of 5.8 mEq/L, morning serum cortisol of 3 µg/dL (N: 10-20 µg/dL), and elevated ACTH levels. Which of the following directly causes one of this patient’s electrolyte disturbances?

A) Hyponatremia due to increased water retention
B) Hyperkalemia due to increased aldosterone secretion
C) Hyponatremia due to decreased cortisol secretion
D) Hyperkalemia due to increased cortisol secretion
E) Hyperkalemia due to decreased cortisol secretion

Detailed Breakdown of Answers + Correct Answer Below ⏬

ANSWER + QUESTION BREAKDOWN

It’s important to adopt the correct MENTAL MODEL when answering USMLE questions; it saves time and increases accuracy. The mental model outlined below is a foundational component of our test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:

Step 1. Read the last line to get to the heart of the question: “Which of the following is a correctly described mechanism behind this patient’s electrolyte disturbances?”

Step 2: Is this a first-, second-, or third-order question?
Answer: 3rd order. 1st: Diagnose the condition; 2nd: Recognize the primary disturbance(s) associated with the diagnosis; 3rd: Identify the correct mechanism provided in the answer choices.

Step 3: Read the vignette carefully and ask yourself: “Based on my diagnosis and the underlying pathology, which of the listed mechanisms is correct?”

Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.

GENERAL ANALYSIS

This vignette describes a 43-year-old woman with signs, symptoms, and lab findings suggesting primary adrenal insufficiency (Addison’s disease). This is characterized by adrenal glands that fail to produce adequate amounts of cortisol and aldosterone.

ANSWER CHOICES:

CHOICE A: Hyponatremia due to increased water retention

Explanation: Cortisol normally inhibits ADH release. In primary adrenal insufficiency, cortisol deficiency leads to increased ADH secretion, causing water retention and dilutional hyponatremia.

CHOICE B: Hyperkalemia due to increased aldosterone secretion

Explanation: Aldosterone promotes potassium excretion in the kidneys. Increased aldosterone would lead to more potassium excretion. This answer is incorrect because aldosterone secretion is decreased in primary adrenal insufficiency, not increased.

CHOICE C: Hyponatremia due to decreased cortisol secretion

Explanation: Decreased cortisol can lead to hyponatremia via increased ADH release, causing water retention. While decreased cortisol does contribute to hyponatremia through increased ADH release, aldosterone deficiency also plays a role by impairing sodium reabsorption. Therefore, this is not the most direct cause of electrolyte abnormality in this scenario.

CHOICE D: Hyperkalemia due to increased cortisol secretion

Explanation: Cortisol does not directly affect potassium regulation like aldosterone does; thus making this option incorrect.

CHOICE E: Hyperkalemia due to decreased cortisol secretion

Explanation: Cortisol does not directly regulate potassium balance; aldosterone does. Hyperkalemia in this patient is due to decreased aldosterone secretion, not cortisol secretion.

FINAL VERDICT…

In primary adrenal insufficiency, there is a deficiency in both cortisol and aldosterone. Aldosterone normally promotes sodium reabsorption and potassium excretion in the kidneys. Its deficiency, therefore, leads to hyponatremia and hyperkalemia. Cortisol deficiency leads to increased release of antidiuretic hormone, contributing to hyponatremia.

CORRECT ANSWER: A) Hyponatremia due to increased water retention