Sorry- I just need to put this here so when I'm on my phone account, writing- I won't forget this information... I guess you guys get a spoiler then...
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A doctor would diagnose the cause of fainting from overworking by first conducting a thorough medical history and physical exam, including blood pressure and heart rate measurements. They would then likely order lab tests (like a complete blood count and blood chemistry panel) and potentially more specialized tests, such as a tilt-table test or an electrocardiogram, to rule out other medical conditions and confirm the diagnosis.