Step 1: Sort the nuclei by their hormones:
Each hypothalamic nucleus is linked to a few hormones or functions, so the answer can be found by matching them.
Supraoptic nucleus: ADH and oxytocin. Paraventricular nucleus: oxytocin, ADH, CRH and TRH. Preoptic area: thermoregulation, sleep and the site of many GnRH cell bodies. Arcuate nucleus: GHRH, dopamine (the PIH) and the KNDy neurons.
Step 2: Eliminate by function:
The supraoptic and paraventricular nuclei are mainly neuroendocrine nuclei for the posterior pituitary and the stress and thyroid axes. Neither one controls reproduction through GnRH pulses, so options (B) and (D) go out.
The preoptic area has GnRH cell bodies and triggers the mid-cycle surge, but a surge is not a pulse generator. So option (A) is not the best choice.
Step 3: Mechanism of the pulse generator:
KNDy neurons of the arcuate nucleus connect to each other. Neurokinin B starts the firing and dynorphin stops it, which gives a rhythm.
Kisspeptin released by them acts on the GnRH neurons, so GnRH comes out as pulses every 60 to 90 minutes in the follicular phase.
Sex steroids, mainly oestrogen and progesterone, act on this population to change the pulse frequency.
Final Answer:
The arcuate nucleus is the site of the GnRH pulse generator. The answer is option (C).