What part of the female reproductive system is sloughed off during menstruation?

The endometrium is the inner lining of the uterus and plays a crucial role in the menstrual cycle. During menstruation, if there is no fertilization of an egg, hormonal changes lead to the shedding of this lining. The shedding is a biological process that prepares the uterus for a new cycle, allowing for the possibility of implantation in the next cycle if fertilization occurs. The loss of the endometrium results in the menstrual flow that is typically experienced each month.

The peritoneum is the membrane that lines the abdominal cavity, but it does not shed during the menstrual cycle. The urethral mucosa is the lining of the urethra, which is irrelevant to menstruation as it pertains to urine passage rather than to the reproductive process. The epimetrium is not a recognized term in relation to female anatomy; it might be confused with the term 'perimetrium,' which refers to the outer layer of the uterus but is also not involved in menstruation. Thus, the correct answer, the endometrium, is specifically related to the menstrual process.