Reproductive Hormones

Which of the following hormones exerts negative feedback on GnRH secretion from the hypothalamus in males?

A 28-year-old woman presents to the gynecology clinic with a 6-month history of amenorrhea. She reports recent weight loss of 8 kg over 3 months, increased stress at work, and denies pregnancy. Physical examination reveals BMI 18.5 kg/m², normal breast development, and normal external genitalia. Serum β-hCG is negative. FSH level is 3.2 mIU/mL (normal follicular phase: 3.5–12.5 mIU/mL), LH is 2.1 mIU/mL (normal: 2.4–12.6 mIU/mL), and prolactin is 12 ng/mL (normal: 4–29 ng/mL). TSH and free T4 are normal. Pelvic ultrasound shows normal uterus and ovaries with no follicles visible. What is the most likely diagnosis?

A 35-year-old man presents to the endocrinology clinic with complaints of erectile dysfunction and decreased libido for 4 months. He denies penile trauma or relationship issues. Physical examination reveals gynecomastia (bilateral breast tissue enlargement), reduced testicular volume (8 mL), and sparse body hair. Laboratory investigations show: total testosterone 180 ng/dL (normal: 300–1000 ng/dL), free testosterone 2.1 pg/mL (normal: 8.3–24 pg/mL), LH 2.4 mIU/mL (normal: 1.7–8.6 mIU/mL), FSH 3.2 mIU/mL (normal: 1.5–12.4 mIU/mL), and prolactin 68 ng/mL (normal: 2–18 ng/mL). MRI pituitary shows a 12 mm sellar mass. What is the most likely diagnosis?

Which enzyme catalyzes the conversion of pregnenolone to 17-hydroxypregnenolone in the adrenal cortex and gonads?