Which statement best describes the mechanism of action of Losartan?

The statement that Losartan is an antagonist of the angiotensin II receptor accurately describes its mechanism of action. As an angiotensin II receptor blocker (ARB), Losartan works by selectively inhibiting the binding of angiotensin II to its receptors, specifically the AT1 receptor. This inhibition leads to vasodilation, reduced secretion of aldosterone, and a decrease in blood pressure, which is particularly beneficial in the treatment of hypertension and heart failure. By blocking the effects of angiotensin II, Losartan mitigates the powerful vasoconstrictive properties of this hormone, helping to improve cardiovascular health.

In contrast, the other options describe different mechanisms that do not pertain to Losartan. For example, inhibiting the renin-angiotensin-aldosterone system encompasses a broader range of actions that may not specifically involve direct receptor antagonism. Blocking calcium channels refers to a different class of medications (calcium channel blockers) that manage hypertension but do so through different pathways. Increasing diuresis by acting on the proximal tubule relates to diuretics rather than ARBs. Thus, option D is the most accurate representation of Losartan's targeted pharmacological action.