The 3 Ps Series (USMLE Step 1 Question)
USMLE Step 1 question based on the "3 Ps"
Today’s question comes from a new series we’re doing called “The 3 Ps Series.” This question will test your fundamental understanding of physiology, pharmacology, pathology, or a mix of all three. Good luck!
A 45-year-old female presents to her primary care physician with recurrent episodes of palpitations and lightheadedness. She denies chest pain, shortness of breath, or syncope. Her medical history is significant for a recent diagnosis of hyperthyroidism, for which she is taking methimazole. Her vital signs are as follows: Blood pressure 130/85 mmHg, heart rate 110/minute, respiratory rate 16/minute, temperature 37.1°C (98.7°F), and oxygen saturation 98% on room air. An ECG reveals episodes of paroxysmal supraventricular tachycardia (PSVT). Which of the following best explains the pathophysiology underlying this patient's symptoms?
A) Increased sodium channel inactivation
B) Decreased adrenergic receptor sensitivity
C) Increased potassium channel activation
D) Decreased potassium channel activation
E) Increased calcium channel inactivation
F) Decreased calcium channel inactivation
G) Increased adrenergic receptor sensitivity
Scroll down for the breakdown ⏬
ANSWER + QUESTION BREAKDOWN
The Mental Model for this week’s question:
Step 1. Identify the goal of the question (ID the pathophysiology of her condition).
Step 2. Make a diagnosis.
Step 3. Ask yourself what underlies these findings.
Step 4. Look for your answer in the answer choices (or one that fits best).
GENERAL ANALYSIS
This question involves a 45-year-old female with hyperthyroidism who presents with recurrent episodes of palpitations and lightheadedness. The ECG reveals paroxysmal supraventricular tachycardia (PSVT). The task is to determine the pathophysiological mechanism underlying her symptoms. Given her recent diagnosis of hyperthyroidism and treatment with methimazole, the focus should be on how these factors contribute to PSVT.
ANSWER CHOICES:
ANSWER CHOICE A: Increased sodium channel inactivation
Sodium channels primarily initiate and propagate action potentials in excitable cells, including cardiac myocytes. Hyperthyroidism generally increases tissues' metabolic rate and excitability but does not specifically cause increased sodium channel inactivation.
ANSWER CHOICE B: Decreased adrenergic receptor sensitivity
Adrenergic receptors mediate the effects of catecholamines like adrenaline and noradrenaline. Hyperthyroidism usually increases adrenergic receptor sensitivity, leading to symptoms like tachycardia.
ANSWER CHOICE C: Increased potassium channel activation
Potassium channels are crucial for the repolarization of the cardiac action potential. Methimazole is an antithyroid drug that inhibits thyroid hormone synthesis but does not directly affect potassium channels.
ANSWER CHOICE D: Decreased potassium channel activation
Similar to option C, methimazole does not directly affect potassium channels. Its role is to reduce thyroid hormone levels by inhibiting thyroid peroxidase.
ANSWER CHOICE E: Increased calcium channel inactivation
Calcium channels play a role in the plateau phase of the cardiac action potential. Hyperthyroidism generally increases calcium influx rather than causing inactivation.
ANSWER CHOICE F: Decreased calcium channel inactivation
This implies that calcium channels remain active longer, which is not typically how hyperthyroidism affects calcium channels. Hyperthyroidism increases calcium channel activity.
ANSWER CHOICE G: Increased adrenergic receptor sensitivity
Hyperthyroidism increases the sensitivity of adrenergic receptors, which leads to increased heart rate and contractility, contributing to symptoms like palpitations and PSVT.
FINAL VERDICT…
Hyperthyroidism results in an excess of thyroid hormones (T3 and T4), significantly affecting the cardiovascular system. These hormones increase the sensitivity of adrenergic receptors, particularly beta-adrenergic receptors, which are involved in the sympathetic nervous system's regulation of heart rate and contractility. The increased adrenergic sensitivity leads to enhanced responsiveness to catecholamines, resulting in increased heart rate, contractility, and cardiac output. This heightened adrenergic activity can precipitate arrhythmias, including paroxysmal supraventricular tachycardia (PSVT), as seen in this patient.
The patient's symptoms of palpitations and lightheadedness, along with the ECG findings of PSVT, are best explained by the increased adrenergic receptor sensitivity due to hyperthyroidism. This mechanism aligns with the known effects of thyroid hormones on the cardiovascular system and the typical presentation of hyperthyroid-induced tachyarrhythmias.
FINAL ANSWER: G: Increased adrenergic receptor sensitivity due to hyperthyroidism
That’s it for today’s question!
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