Which hormones antagonize the effect of NO and cause the penis to become flaccid after orgasm?
**Core Concept**
The question is testing the physiological mechanism of erection and ejaculation in males, particularly the role of nitric oxide (NO) and its antagonistic hormones. The underlying principle involves the balance between vasodilation and vasoconstriction in the penile arteries, which is crucial for maintaining an erection.
**Why the Correct Answer is Right**
Nitric oxide (NO) is a potent vasodilator that stimulates the production of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation and increased blood flow to the penis. However, after orgasm, the hormone **oxytocin** and **vasopressin** (also known as antidiuretic hormone, ADH) counteract the effects of NO by causing vasoconstriction and reducing blood flow to the penis. This leads to detumescence and flaccidity. Additionally, **oxytocin** also stimulates the contraction of the vas deferens and seminal vesicles, facilitating ejaculation.
**Why Each Wrong Option is Incorrect**
* **Option A:** **Adrenaline** (epinephrine) can cause vasoconstriction, but it is not the primary hormone responsible for flaccidity after orgasm.
* **Option B:** **Estrogen** has various effects on the male reproductive system, but it is not directly involved in the antagonism of NO after orgasm.
* **Option C:** **Testosterone** is essential for male libido and fertility, but it does not directly cause vasoconstriction or flaccidity after orgasm.
**Clinical Pearl / High-Yield Fact**
Remember the "NO-Go" principle: after orgasm, the body's "NO" (nitric oxide) response is countered by "oxytocin" and "vasopressin," leading to detumescence and flaccidity.
**Correct Answer:** D.