**Core Concept**
Aldosterone antagonists, such as spironolactone and eplerenone, are a class of potassium-sparing diuretics that act by blocking the mineralocorticoid receptor (MR) in the collecting duct of the kidneys. This receptor is primarily activated by aldosterone, leading to increased sodium reabsorption and potassium excretion.

**Why the Correct Answer is Right**
Aldosterone antagonists are useful in treating conditions such as primary aldosteronism (Conn's syndrome), resistant hypertension, and heart failure. They work by counteracting the effects of excess aldosterone, thereby reducing sodium retention and blood pressure. Additionally, they can improve cardiac function and reduce mortality in patients with heart failure by reducing fibrosis and improving ejection fraction.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because aldosterone antagonists are not useful in treating hyperkalemia, as they can actually worsen potassium levels by reducing potassium excretion.
**Option B:** This choice is incorrect because aldosterone antagonists can be useful in treating certain cases of liver cirrhosis with ascites, where they help reduce sodium retention and ascites formation.
**Option C:** This choice is incorrect because aldosterone antagonists can be useful in treating primary aldosteronism (Conn's syndrome), which is a condition characterized by excess aldosterone production.

**Clinical Pearl / High-Yield Fact**
Aldosterone antagonists can cause gynecomastia, a side effect due to their anti-androgenic properties. This is an important consideration when prescribing these medications, especially for male patients.

**Correct Answer: D. Hyperkalemia.**