A 26 year old woman with regular menses and her 28 year old partner comes to the GP surgery complaining of primary infertility for 2 years. What would be the single best investigation to see whether she is ovulating or not?

The key is D. Day 21 progesterone. [Mid-luteal progesterone level to assess ovulation: If low, it may

need repeating, as ovulation does not occur every month. The blood test is taken seven days before the

anticipated period, that is on day 21 of a 28-day cycle. However, this day will need to be adjusted for

different lengths of cycle. Ref: patient.co.uk].