**Question:** A 38 year old nullipara is on treatment for infertility. Inhibin B levels are tested to assess ovarian reserve. Inhibin B level is best measured which day of menstruation:

A. Day 2
B. Day 7
C. Day 10
D. Day 14

**Core Concept:** Inhibin B is a hormone secreted by the granulosa cells of the follicles within the ovaries. It plays a role in assessing ovarian reserve, which is important in evaluating a patient's fertility potential.

**Why the Correct Answer is Right:** Inhibin B levels are influenced by the number and health of the ovarian follicles, which are most active during the follicular phase of the menstrual cycle. The peak of Inhibin B secretion occurs around the mid-follicular phase, typically around day 10-12 of a typical 28-day menstrual cycle. Hence, the best time to measure Inhibin B levels to assess ovarian reserve is around day 10 to day 14 of the menstrual cycle.

**Why Each Wrong Option is Incorrect:**

A. Day 2: Inhibin B levels are highest during the early follicular phase, but measuring on day 2 would be too early as the follicular phase has not yet begun.

B. Day 7: Measuring on day 7 is too late as the peak of Inhibin B secretion occurs around day 10-14.

C. Day 10: Although close to the peak, measuring on day 10 is still slightly early, as the peak occurs around day 10-12.

D. Day 14: Measuring on day 14 is too late, as the peak of Inhibin B secretion has passed around day 10-12, making it an inappropriate day to assess ovarian reserve.

**Clinical Pearl:** Assessing Inhibin B levels during the mid-follicular phase (day 10-14) can provide valuable information regarding a patient's ovarian reserve, which is crucial in evaluating their fertility potential and guiding infertility treatment decisions.