**Core Concept**
The child in question is likely suffering from congenital hypertrophic cardiomyopathy (CHC), a condition characterized by abnormal thickening of the heart muscle, particularly the septal wall. This condition can lead to obstructive symptoms, such as tachypnea and hepatomegaly, due to increased resistance to blood flow.

**Why the Correct Answer is Right**
The treatment of choice for CHC is a beta-blocker, specifically propranolol. Beta-blockers work by reducing the heart rate and contractility, thereby decreasing the thickness of the septal wall and alleviating the obstruction. This is particularly effective in infants and young children with CHC. The use of beta-blockers in CHC is based on the principle of reducing the heart's metabolic demands and improving cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** Digoxin is not typically used in the treatment of CHC, as it can actually increase the contractility of the heart muscle, exacerbating the obstruction.
**Option B:** Diuretics may be used to manage symptoms such as fluid overload, but they do not address the underlying cause of CHC.
**Option C:** ACE inhibitors are not typically used in CHC, as they can actually decrease cardiac output and worsen symptoms.

**Clinical Pearl / High-Yield Fact**
Beta-blockers are the primary treatment for congenital hypertrophic cardiomyopathy in infants and young children, and should be initiated promptly to alleviate symptoms and prevent long-term cardiac damage.

**Correct Answer:** B. Metoprolol.