**Core Concept**
Losartan, commonly known by its brand name losaan, is an angiotensin II receptor antagonist (ARB) used to treat hypertension and heart failure. ARBs block the action of angiotensin II on its receptor, leading to vasodilation and decreased blood pressure.

**Why the Correct Answer is Right**
Angioedema is a rare but potentially life-threatening side effect of ARBs like losartan. It occurs due to the increased levels of bradykinin, a peptide that causes blood vessels to dilate, leading to swelling, particularly in the face, lips, tongue, or larynx. This is more likely to occur in patients with a history of angiotensin-converting enzyme (ACE) inhibitor use. Losartan, being an ARB, is less likely to cause angioedema compared to ACE inhibitors.

**Why Each Wrong Option is Incorrect**
**Option B:** Cough is a common side effect of ACE inhibitors, not ARBs like losartan. ACE inhibitors can cause cough due to the increased levels of bradykinin, which stimulates the cough reflex.

**Option C:** Hyperkalemia is a potential side effect of ARBs, particularly in patients with renal impairment. Losartan can cause hyperkalemia by blocking the renin-angiotensin-aldosterone system (RAAS), leading to decreased potassium excretion.

**Option D:** Headache is a common side effect of various medications, including ARBs like losartan. The exact mechanism of losartan-induced headache is unclear, but it is thought to be related to vasodilation and changes in blood pressure.

**Clinical Pearl / High-Yield Fact**
When switching patients from ACE inhibitors to ARBs, be aware of the increased risk of hyperkalemia, particularly in patients with renal impairment. Monitor potassium levels closely and consider dose adjustments or alternative medications as necessary.

**✓ Correct Answer: A. Angioedema**