A new patient who is an athlete has a resting pulse of 48 bpm and is otherwise well. What is the most likely explanation?

Correct answer: B. Physiologic bradycardia from athletic conditioning

• A. Incorrect. Hypothyroidism causes bradycardia but with fatigue, weight gain, cold intolerance and dry skin.
• B. Correct. Endurance training increases stroke volume and vagal tone, so a lower heart rate maintains the same cardiac output. Resting rates of 40–60 bpm are normal in trained athletes and require no modification.
• C. Incorrect. Possible in general, but the question specifies an athletic patient with no other findings.
• D. Incorrect. Pathologic bradycardia would be accompanied by symptoms such as dizziness, syncope, fatigue or exercise intolerance.