Final answer:
Furosemide is the drug that will decrease a patient's KCl level because it is a loop diuretic that increases urination and potassium excretion.
Step-by-step explanation:
The drug that will cause a decrease in a patient's KCl (potassium chloride) level is Furosemide. Furosemide is a loop diuretic and is well-known for its ability to treat hypertension by inhibiting the reabsorption of Na+ and Cl- ions in the ascending limb of the loop of Henle. This process leads to an increase in urination, which in turn causes more potassium to be excreted, potentially leading to decreased potassium levels, or hypokalemia. Spironolactone and Triamterene, on the other hand, are potassium-sparing diuretics and are less likely to cause a decrease in potassium levels. Tamsulosin is a medication used to treat urinary symptoms but does not typically affect potassium levels.