**Core Concept**
Hemochromatosis is a genetic disorder characterized by excessive iron accumulation in the body, leading to oxidative stress and tissue damage. Cardiomyopathy is a common complication of hemochromatosis due to iron deposition in the heart muscle.

**Why the Correct Answer is Right**
The investigation of choice to confirm hemochromatosis as the cause of cardiomyopathy is a liver biopsy, which allows for the measurement of iron stores in the liver. This is crucial as hemochromatosis is typically diagnosed by demonstrating increased iron stores in the liver. The liver biopsy also helps to rule out other causes of cardiomyopathy. The iron stores in the liver can be quantified using the **siderotic index**, which is a key diagnostic criterion for hemochromatosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum ferritin levels are elevated in hemochromatosis, but they can also be elevated in other conditions such as iron deficiency anemia, acute phase reactions, and malignancies. Therefore, serum ferritin levels are not specific for diagnosing hemochromatosis.

**Option B:** Cardiac MRI can show iron deposition in the heart muscle, but it is not the investigation of choice for diagnosing hemochromatosis. Cardiac MRI is useful for assessing the extent of cardiac iron deposition, but it does not provide a definitive diagnosis of hemochromatosis.

**Option C:** Genetic testing for HFE gene mutations is useful for identifying individuals at risk of developing hemochromatosis, but it is not the investigation of choice for confirming hemochromatosis as the cause of cardiomyopathy. Genetic testing can identify individuals with a genetic predisposition to hemochromatosis, but it does not provide a definitive diagnosis of the condition.

**Clinical Pearl / High-Yield Fact**
In patients with cardiomyopathy, it is essential to consider hemochromatosis in the differential diagnosis, particularly if there is a family history of hemochromatosis or if the patient has other symptoms such as skin pigmentation or diabetes mellitus.

**Correct Answer:** C. Liver biopsy