5% guanethidine is used for
**Core Concept**
Guanethidine is a centrally acting antihypertensive drug that works by inhibiting the release of norepinephrine from postganglionic sympathetic neurons. It is used to treat hypertension, particularly in cases where other antihypertensive agents have failed.
**Why the Correct Answer is Right**
The 5% solution of guanethidine is used topically to induce miosis (pupil constriction) before ocular surgery, as it temporarily paralyzes the iris sphincter muscle. This results in a dilated pupil on the side of the surgery, allowing the surgeon to operate without the risk of the pupil constricting during the procedure. The mechanism of action involves the inhibition of norepinephrine release, which normally stimulates the iris sphincter muscle to contract.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because guanethidine is not typically used to treat glaucoma, although its mechanism of action could theoretically be beneficial in reducing intraocular pressure.
**Option B:** This is incorrect because guanethidine is not used to treat conditions related to the autonomic nervous system, such as orthostatic hypotension.
**Option C:** This is incorrect because guanethidine is not used to treat conditions related to the cardiovascular system, such as heart failure.
**Clinical Pearl / High-Yield Fact**
It is essential to note that guanethidine has a long duration of action and can cause significant postural hypotension, making it unsuitable for use in patients with severe hypertension or those taking other antihypertensive medications.
**Correct Answer: D. Topical use in ophthalmology.