A young woman is given daily injections of a substance beginning on the sixteenth day of her normal menstrual cycle and continuing for 3 weeks. As long as the injections continue, she does not menstruate. The injected substance could be which of the following?
**Core Concept**
The underlying principle here is the pharmacological manipulation of the menstrual cycle, specifically the suppression of gonadotropin secretion and subsequent prevention of ovulation. This is achieved through the administration of a substance that mimics the action of gonadotropin-releasing hormone (GnRH) or directly inhibits the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
**Why the Correct Answer is Right**
The injected substance is likely a GnRH agonist, which initially stimulates the pituitary gland to release LH and FSH. However, with prolonged exposure to the agonist, the pituitary gland downregulates its receptors, leading to a decrease in LH and FSH secretion. This reduction in gonadotropin levels causes the ovaries to stop producing estrogen, resulting in a decrease in follicular growth and subsequent amenorrhea (absence of menstruation). This mechanism is often used in the treatment of endometriosis, uterine fibroids, and precocious puberty.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly relate to the pharmacological manipulation of the menstrual cycle. While it may be involved in the treatment of other conditions, it is not the substance that would cause amenorrhea through the mechanism described.
**Option B:** This option is incorrect because it is a type of hormonal contraceptive that works by thickening the cervical mucus and thinning the uterine lining, but it does not directly affect gonadotropin secretion.
**Option C:** This option is incorrect because it is an antifertility agent that works by disrupting the normal transport of sperm through the cervix, but it does not cause amenorrhea through the suppression of gonadotropin secretion.
**Clinical Pearl / High-Yield Fact**
GnRH agonists can cause a temporary increase in bone resorption due to the initial surge in LH and FSH levels, which can lead to osteoporosis in long-term users. This is an important consideration when selecting patients for treatment with GnRH agonists.
**Correct Answer:** C.