**Core Concept**
The lung plays a crucial role in the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure and fluid balance. The conversion of angiotensin-I to angiotensin-II is a key step in this system, leading to vasoconstriction and increased blood pressure.

**Why the Correct Answer is Right**
The conversion of angiotensin-I to angiotensin-II occurs through the action of angiotensin-converting enzyme (ACE), which is primarily located in the pulmonary capillary endothelium. ACE is a metalloprotease that cleaves the C-terminal dipeptide from angiotensin-I, resulting in the formation of angiotensin-II. Angiotensin-II is a potent vasoconstrictor that also stimulates the release of aldosterone from the adrenal cortex, leading to increased sodium retention and water reabsorption in the kidneys. This ultimately results in increased blood pressure and fluid balance.

**Why Each Wrong Option is Incorrect**
**Option A:** Inactivation of serotonin occurs in the liver and other tissues, but it is not a primary function of the lung.

**Option C:** Inactivation of bradykinin occurs through the action of ACE, but this is not the primary metabolic function of the lung. Bradykinin is a vasodilator that counteracts the effects of angiotensin-II.

**Option D:** Metabolism of basic drugs by cytochrome P-450 system occurs primarily in the liver, not the lung.

**Clinical Pearl / High-Yield Fact**
The RAAS plays a critical role in regulating blood pressure and fluid balance. Understanding the lung's role in converting angiotensin-I to angiotensin-II is essential for managing conditions such as hypertension and heart failure.

✓ Correct Answer: B. Conversion of angiotensin–I to angiotensin–II