[Solved] The physiological trigger for the onset of menstruation in humans is caused by A) ovulation. B) an increase in estrogen. C) the secretion of progesterone by the corpus luteum. D) degeneration of the corpus luteum, resulting in a lack of progesterone for the support of the endometrium. Systemic Anatomy: The study of the body's systems, looking at how various organs and structures function together. Molecular Biology: A branch of biology that focuses on the structure and function of the molecules that make up cells, particularly DNA, RNA, and proteins. Microbiology: The branch of science concerned with microorganisms and their effects on other living organisms. Surface Anatomy: The study of the external features of the body and their relationship to deeper structures, as visible to the naked eye without dissection.