#71 - THE WEEKLY USMLE TIMES
Highlights for the week of April 15, 2024
Welcome to this week’s issue of The USMLE Times… A special welcome to the 134 new members who have joined our community this week.
Here’s what’s on tap for issue #71 of the USMLE Times:
Question of the Week
Video lessons of the week
Tweet of the week
Links to this week’s training
Question deep-dive & breakdown
Let’s dive in!
A 28-year-old female presents to the emergency department with acute-onset muscle weakness and difficulty breathing. She reports a recent episode of severe vomiting and diarrhea after eating at a local restaurant. She is otherwise healthy and takes no medications. Her vital signs are as follows: Blood pressure 110/70 mmHg, heart rate 95/minute, respiratory rate 18/minute, temperature 98.6°F (37°C), and oxygen saturation 97% on room air. An ECG shows a prominent U wave and flattened T waves. Which of the following best explains the pathophysiology underlying this patient's symptoms?
A. Increased extracellular sodium concentration leading to hyperpolarization of the membrane potential
B. Increased extracellular potassium concentration leading to depolarization of the membrane potential
C. Decreased extracellular sodium concentration leading to depolarization of the membrane potential
D. Decreased intracellular chloride concentration leading to hyperpolarization of the membrane potential
E. Increased intracellular calcium concentration leading to depolarization of the membrane potential
F. Decreased extracellular potassium concentration leading to hyperpolarization of the membrane potential
G. Decreased intracellular calcium concentration leading to hyperpolarization of the membrane potential
The answer & question breakdown is at the bottom of the post.
📺 WATCH
📺 WATCH
💬 TWEET FOR THOUGHT
Click HERE To Follow Us On Twitter/X
🔗 LINKS TO THIS WEEK’S TRAINING
April 15 - Genetics Drill Session - CLICK HERE
April 16 - Step 1 Hematology Question - CLICK HERE
April 18 - Ask Dr. Paul (Two years studying, still not ready) - CLICK HERE
ANSWER + QUESTION BREAKDOWN
This question assesses your understanding of electrolyte disturbances and their effects on membrane potential and muscle function. The patient's severe vomiting and diarrhea, muscle weakness, difficulty breathing, and ECG findings suggest an electrolyte imbalance. Your job is to consider which electrolyte disturbance best explains the patient's symptoms and ECG changes.
ANSWER CHOICES:
ANSWER CHOICE A: Increased extracellular sodium concentration leading to hyperpolarization of the membrane potential
Increased extracellular sodium would cause hypernatremia, leading to hyperpolarization and muscle weakness. However, this is unlikely given the patient's history of vomiting and diarrhea.
ANSWER CHOICE B: Increased extracellular potassium concentration leading to depolarization of the membrane potential
Increased extracellular potassium would cause hyperkalemia, leading to depolarization. This can cause muscle weakness, but the ECG would show peaked T waves and widened QRS complexes, not prominent U waves and flattened T waves.
ANSWER CHOICE C: Decreased extracellular sodium concentration leading to depolarization of the membrane potential
Decreased extracellular sodium would cause hyponatremia, leading to membrane depolarization. This can cause muscle weakness but would not explain the ECG findings of prominent U waves and flattened T waves.
ANSWER CHOICE D: Decreased intracellular chloride concentration leading to hyperpolarization of the membrane potential
Decreased intracellular chloride is not a common electrolyte disturbance and would not directly affect the membrane potential or explain the patient's symptoms and ECG findings.
ANSWER CHOICE E: Increased intracellular calcium concentration leading to depolarization of the membrane potential
Increased intracellular calcium would cause muscle contraction, not weakness. It would also not explain the ECG findings.
ANSWER CHOICE F: Decreased extracellular potassium concentration leading to hyperpolarization of the membrane potential
Decreased extracellular potassium would cause hypokalemia, leading to hyperpolarization of the membrane potential. This can cause muscle weakness, and the ECG findings of prominent U waves and flattened T waves are characteristic of hypokalemia.
ANSWER CHOICE G: Decreased intracellular calcium concentration leading to hyperpolarization of the membrane potential
Decreased intracellular calcium would cause muscle weakness but would not explain the ECG findings.
THE VERDICT: The patient's severe vomiting and diarrhea can lead to hypokalemia due to potassium through the GI tract. Hypokalemia causes hyperpolarization of the cell membrane, making it harder for the cell to reach the threshold potential for muscle contraction, leading to muscle weakness and difficulty breathing. The ECG findings of prominent U waves and flattened T waves also characterize hypokalemia. Therefore, decreased extracellular potassium concentration best explains the patient's symptoms and ECG findings.
FINAL ANSWER: F: Decreased extracellular potassium concentration leading to hyperpolarization of the membrane potential.
That’s all for issue #71 of The USMLE Times!
If you enjoyed it and found it helpful, please consider hitting that little heart button, telling me if you got the answer right in the comments, and sharing it with someone you know who would also benefit from it (use the little pink ‘share’ button below).
Thanks for tuning in!
If you’re struggling with med school or USMLE prep and need a helping hand, book a one-on-one session with me HERE.
