Respected sir/madam,
I am ________ (name) and I am working as _______ (designation) in your reputed company. My employee ID number is __________ (mention your employee ID).
I write the email most humbly to state that I am willing to take casual leave for __/__/_____ (date) as I have to get my medical check done. My health checkup is scheduled for __/__/_____ (date) and for which I have to be presented at the _________ (lab centre/clinic/hospital/other).
Respected, the same being important and urgent. I request you to kindly sanction leave for the above-said date. As it cannot be rescheduled, I expect to hear back from you at the earliest.
Thanking you,
Sincerely,
__________ (Name),
__________ (Employee ID)