**Core Concept**
Metabolic alkalosis is a condition characterized by an increase in blood pH due to an excess of bicarbonate ions. The kidneys play a crucial role in managing this condition by adjusting the excretion of various substances to compensate for the alkalosis. The kidneys reabsorb bicarbonate and can also excrete excess hydrogen ions to help normalize the pH balance.

**Why the Correct Answer is Right**
In metabolic alkalosis, the kidneys aim to conserve bicarbonate and eliminate excess hydrogen ions. To do this, they decrease the excretion of ammonia (NH3), which is a buffer that helps to neutralize excess hydrogen ions. By reducing ammonia excretion, the kidneys can help to increase the concentration of hydrogen ions in the urine, which in turn helps to compensate for the alkalosis. This is achieved through the action of the enzyme glutaminase, which breaks down glutamine to form ammonia, which is then excreted in the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** More of NH3 would actually worsen the alkalosis by increasing the concentration of bicarbonate in the urine.

**Option B:** The question is asking about the excretion of aceto-acetic acid, but this is not directly related to metabolic alkalosis. Aceto-acetic acid is a ketone body, and its excretion is more relevant to diabetic ketoacidosis.

**Option C:** Betahydroxy butyric acid is a ketone body, and its excretion is also not directly related to metabolic alkalosis.

**Clinical Pearl / High-Yield Fact**
In metabolic alkalosis, the kidneys also increase the excretion of potassium ions (K+) to help compensate for the alkalosis. This is because potassium ions are often retained in exchange for hydrogen ions in the kidneys, and in metabolic alkalosis, the kidneys aim to eliminate excess hydrogen ions.

**✓ Correct Answer: D. Less ammonia.**