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

Inhibins are produced by Seoli cells in males and by granulosa cells in females. Serum inhibin levels vary widely across the menstrual cycle. During the early follicular phase, FSH stimulates the secretion of inhibin B by the granulosa cells. However, increasing levels of circulating inhibin B blunt FSH secretion later in the follicular phase. A low level of inhibin B on day 3 may indicate a declining ovarian reserve which is helpful for assessing reproductive ageing and ovarian reserve. During the luteal phase, regulation of inhibin production comes under the control of LH and switches from inhibin B to inhibin A. Inhibin B levels peak during the midluteal phase, decrease with the loss of luteal function, and remain low during the luteal-follicular transition and early follicular phase. The inverse relationship between circulating inhibin levels and FSH secretion is consistent with a negative-feedback role for inhibin in regulating FSH secretion. Ref: Hoffman B.L., Schorge J.O., Schaffer J.I., Halvorson L.M., Bradshaw K.D., Cunningham F.G., Calver L.E. (2012). Chapter 15. Reproductive Endocrinology. In B.L. Hoffman, J.O. Schorge, J.I. Schaffer, L.M. Halvorson, K.D. Bradshaw, F.G. Cunningham, L.E. Calver (Eds), Williams Gynecology, 2e.