Although, I have enclosed the letter format, please be prepared to be questioned by the HOD about why the check-up has to happen exactly on the day of the sesssional exams and also anticipate him/her asking for a letter of support from mother's treating doctor.
The Head of Department,
Subject: Request to grant leave of abscence from the sessional exams.
This is to update your kind self regarding my predictament related to the sessional exams this semster. My mother is suffering from X,Y and she needs her medical consultation and investigations as part of the treatment plan. Unforutnately, this check-up is carried our by the specialists on particular days and my mother's appointment clashes with the exams dates. Due to unaviodable circumstances, I am the only person who is around to take my mother of her health care needs. This is more important at this point of time. I would request your kind self to allow me to take care of my mother's health by granting me leave on the days -----.
I assure you that, I will take the exams prior or after these days in the college.
Name, Year, Branch/Division.
Please find enclosed the dcotor's request/appointment booked at the hospital.