THE USMLE TIMES - #88
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Welcome to issue #88 of The USMLE Times… This is the 88th straight week we’ve posted our USMLE Times Newsletter! Thank you to all of our supporters—new and old!
I’m Paul Ciurysek, MD, and this is The USMLE Times: An independent, subscriber-supported newsletter designed to help you survive medical school, pass your USMLE exams, and match into your dream residency program.
Here’s what’s on tap for issue #88 of the USMLE Times:
Question of the Week (Membrane physiology)
This week’s video training (Life-Changing Advice For Med Students)
Question deep-dive & breakdown
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Let’s dive in!
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A 20-year-old female college student visits the campus clinic with complaints of muscle weakness, fatigue, and episodes of muscle cramping. She has been training for a marathon and recently increased her exercise intensity. She reports consuming a sports drink after post-workout to replenish her electrolytes. The physician orders laboratory tests, which reveal low serum potassium levels. The physician counsels her on dietary changes and recommends further evaluation. Which of the following cell membrane physiology changes would be expected in the patient's muscle cells in response to her hypokalemia?
A. Increased resting membrane potential
B. Decreased resting membrane potential
C. Increased action potential duration
D. Decreased action potential duration
E. Increased threshold for action potential generation
F. Decreased threshold for action potential generation
G. No change in membrane potential or action potential generation
The answer & question breakdown is at the bottom of the post.
🔗 LINKS TO RECENT POSTS
USMLE Times Issue #87 - READ IT HERE
USMLE Times Issue #86 - READ HERE
USMLE Times Issue #85 - READ HERE
ANSWER + QUESTION BREAKDOWN
The Mental Model for this week’s question:
Step 1. Identify the question’s goal (Change expected in muscle cells in response to hypokalemia).
Step 2. Read the vignette to better understand the question’s context.
Step 3. Ask yourself what would happen in the presence of this abnormality.
Step 4. Look to the answer choices and find the answer that most closely resembles your hypothesis (Trust your instincts; look for your hypothesis from “step 3” above).
GENERAL ANALYSIS
The question involves a 20-year-old female college student who experiences muscle weakness, fatigue, and cramping after increasing her exercise intensity while training for a marathon. Laboratory tests reveal hypokalemia). The task is determining the expected change in cell membrane physiology in her muscle cells due to hypokalemia. Note: This is a “basics of membrane physiology” question wrapped in a clinical scenario—see it for what it is!
ANSWER CHOICES:
ANSWER CHOICE A: Increased resting membrane potential
An increase in resting membrane potential means the inside of the cell becomes less negative (depolarized).
ANSWER CHOICE B: Decreased resting membrane potential
A decrease in resting membrane potential means the inside of the cell becomes more negative (hyperpolarized).
ANSWER CHOICE C: Increased action potential duration
Increased action potential duration means the action potential lasts longer.
ANSWER CHOICE D: Decreased action potential duration
Decreased action potential duration means the action potential is shorter.
ANSWER CHOICE E: Decreased threshold for action potential generation
An increased threshold means a stronger stimulus is needed to generate an action potential.
ANSWER CHOICE F: Decreased threshold for action potential generation
A decreased threshold means a weaker stimulus is needed to generate an action potential.
ANSWER CHOICE G: No change in membrane potential or action potential generation
This would mean hypokalemia has no effect on membrane physiology.
THE VERDICT…
Hypokalemia refers to a decrease in extracellular potassium concentration, which increases the gradient for potassium to leave the cell. This results in more potassium exiting the cell, making the inside of the cell more negative, thereby hyperpolarizing the resting membrane potential.
Hyperpolarization makes it more difficult for the cell to reach the threshold needed to generate an action potential, effectively increasing the threshold for action potential generation.
This hyperpolarization of the resting membrane potential is a key physiological change in muscle cells during hypokalemia, leading to symptoms such as muscle weakness and cramping. The decreased resting membrane potential is the primary change in cell membrane physiology due to hypokalemia
FINAL ANSWER: B: Decreased resting membrane potential
That’s it for issue #88 of The USMLE Times!
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