**Core Concept**
Angiotensin II type 1 (AT1) receptors play a crucial role in the renin-angiotensin-aldosterone system (RAAS), mediating vasoconstriction and aldosterone secretion. Blockade of AT1 receptors leads to vasodilation and reduced aldosterone levels, which is beneficial in treating hypertension and heart failure.

**Why the Correct Answer is Right**
Losartan is an angiotensin II receptor antagonist (ARB) that selectively blocks the AT1 receptor, preventing angiotensin II from binding and inducing vasoconstriction and aldosterone secretion. This mechanism of action results in decreased peripheral resistance, reduced blood pressure, and decreased fluid retention. Losartan is primarily used to treat hypertension and diabetic nephropathy.

**Why Each Wrong Option is Incorrect**
**Option A:** Spironolactone is a potassium-sparing diuretic that acts as an aldosterone antagonist, blocking the action of aldosterone at the mineralocorticoid receptor, not the AT1 receptor. It is primarily used to treat hypertension, heart failure, and primary aldosteronism.
**Option C:** Captopril is an angiotensin-converting enzyme (ACE) inhibitor that blocks the conversion of angiotensin I to angiotensin II, thereby reducing the amount of angiotensin II available to bind to AT1 receptors. However, it does not directly block the AT1 receptor.

**Clinical Pearl / High-Yield Fact**
It is essential to distinguish between ARBs (like losartan) and ACE inhibitors (like captopril), as they have different mechanisms of action and potential side effects. ARBs are often preferred in patients with chronic kidney disease or those who are intolerant to ACE inhibitors.

**✓ Correct Answer: B. Losartan**