Sleep.
My favorite class in undergrad was Sleep: Neurobiology, Medicine, and Society. https://www.coursera.org/learn/sleep. There were a handful of fellows/residents that would sit in on lectures.
I now work in nursing, with a BSN curriculum that only had 12 lectures slides on sleep whatsoever (i.e. sleep apnea bad, older adults wake up earlier, pain and urinary frequency impair sleep). The patient population I see (CVICU), is extremely likely to have OSA. Their other comorbidities are unsurprising-CAD, HTN, Hld, anxiety, obesity, etc. I'm seeing patients late in the game when they need heart transplants, VADs, ECMO, and addressing impaired sleep is not a priority.
This is a passion project of mine, to make nursing more aware of disordered sleep, how to assess for it, and how to promote healthy sleep. I invite anyone with a stake in preventing/managing chronic illness to consider impaired sleep as a principle contributor to poor outcomes, albeit a very modifiable factor.
How much was this subject covered in your med school program?