A 57-year-old man comes to the emergency department with severe right-sided eye pain and ipsilateral headache. Furthermore, the patient reports severe nausea and describes seeing "halos" around objects. After initial treatment with the appropriate medication, the severity of his pain decreases. He also experiences increased diuresis with highly alkaline urine. The drug used to treat this patient's eye condition predominantly acts on which of the following nephron segments?

Question

A 57-year-old man comes to the emergency department with severe right-sided eye pain and ipsilateral headache.  Furthermore, the patient reports severe nausea and describes seeing "halos" around objects.  After initial treatment with the appropriate medication, the severity of his pain decreases.  He also experiences increased diuresis with highly alkaline urine.  The drug used to treat this patient's eye condition predominantly acts on which of the following nephron segments?

Quizplus · Accepted Answer

This patient has acute angle-closure glaucoma and has been treated with acetazolamide. Acetazolamide is a diuretic that works by inhibiting the enzyme carbonic anhydrase. Carbonic anhydrase is found in high concentrations in the proximal tubule and is responsible for catalyzing reactions necessary for NaHCO₃ reabsorption. By inhibiting carbonic anhydrase, acetazolamide and other carbonic anhydrase inhibitors effectively block HCO₃^- reabsorption in the proximal tubules. This results in enhanced HCO₃^- and water excretion as well as increased urinary pH and potential metabolic acidosis. Carbonic anhydrase is also present in the eyes, pancreas, gastrointestinal tract, CNS, and red blood cells. In eye tissues, carbonic anhydrase modulates HCO₃^- formation in the aqueous humor. Inhibition of carbonic anhydrase will decrease HCO₃^- and aqueous humor formation; thus, a number of carbonic anhydrase inhibitors are used to relieve intraocular pressures in open-angle and angle-closure glaucoma. Common side effects of carbonic anhydrase inhibitors include somnolence, paresthesias, and urine alkalinization. Rare side effects include metabolic acidosis, dehydration, hypokalemia, and hyponatremia. (Choice B) The descending limb of the loop of Henle carries fluid from the proximal tubule to the ascending limb of the loop of Henle in the medulla. The descending limb is very permeable to water, allowing water to diffuse into interstitial fluids to produce a more concentrated tubular fluid. (Choice C) Loop diuretics work in the thick ascending limb and are the most potent diuretics. (Choice D) The distal tubule actively transports Na⁺ and Cl^- and is impermeable to water. Thiazide diuretics work in the distal tubule. (Choice E) The collecting duct system includes the collecting tubules and ducts. Here, aldosterone and ADH make final adjustments to electrolytes and water content. Potassium-sparing diuretics and aldosterone antagonists also work in the collecting duct. Educational objective: Carbonic anhydrase is found in high concentrations in the proximal tubule and is responsible for catalyzing reactions necessary for NaHCO₃ reabsorption. Acetazolamide is a diuretic that works by inhibiting carbonic anhydrase, which effectively blocks NaHCO₃ and water reabsorption in the proximal tubules resulting in urinary bicarbonate wasting. Carbonic anhydrase inhibitors are also used to relieve intraocular pressure in open-angle and angle-closure glaucoma. __________ References: Ophthaproblem. Acute angle-closure glaucoma. (http://www.ncbi.nlm.nih.gov/pubmed/10690485)

A 57-Year-Old Man Comes to the Emergency Department with Severe