A 26 year old woman with 32-day menstrual cycle planning for conception regularly checks her body temperature (orally) using a digital thermometer in the morning before getting out of bed. She noticed an increase in her body temperature on the 18th day of menstrual cycle. This is caused by
**Core Concept**
The question is testing the understanding of the physiological changes that occur during the menstrual cycle, specifically the ovulatory phase and the associated increase in basal body temperature (BBT). This phenomenon is a key indicator of ovulation and is often used to track fertility.
**Why the Correct Answer is Right**
The increase in basal body temperature on the 18th day of the menstrual cycle is due to the release of progesterone after ovulation. Progesterone is a hormone produced by the corpus luteum, a temporary endocrine structure that forms in the ovary after ovulation. As progesterone levels rise, the body temperature increases by about 0.4°F (0.2°C) due to its thermogenic effect. This temperature shift is a reliable indicator of ovulation and is often used in fertility tracking methods.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because ovulation itself does not directly cause an increase in basal body temperature. The increase in temperature is a result of the hormonal changes that occur after ovulation.
* **Option B:** This option is incorrect because the increase in basal body temperature is not caused by the luteinizing hormone (LH) surge, although LH does trigger ovulation. The LH surge is a sudden increase in LH levels that occurs just before ovulation, but it does not directly cause the temperature increase.
* **Option C:** This option is incorrect because the follicle-stimulating hormone (FSH) is involved in follicular growth and ovulation, but it does not directly cause the increase in basal body temperature.
**Clinical Pearl / High-Yield Fact**
It's essential to note that basal body temperature is a valuable tool for fertility tracking, but it should be used in conjunction with other methods, such as ovulation predictor kits (OPKs) and cervical mucus observations, for accurate ovulation prediction.
**Correct Answer: C.**