**Core Concept**
Cardiac muscle cells, like other mammalian myocytes, rely on a specific energy source to maintain their high contractile activity. Under normal conditions, the heart primarily utilizes fatty acids as its preferred fuel source.

**Why the Correct Answer is Right**
Fatty acids serve as the primary energy source for cardiac muscle cells under normal conditions due to their high energy yield and efficient metabolic pathway. The heart's high energy demands are met by the beta-oxidation of fatty acids, which produces ATP through the citric acid cycle and oxidative phosphorylation. This process is facilitated by the presence of a high density of mitochondria in cardiac muscle cells, allowing for efficient fatty acid oxidation.

**Why Each Wrong Option is Incorrect**
**Option B:** Ketone bodies are an alternative energy source for the heart, particularly during periods of fasting or low glucose availability. However, they are not the primary energy source under normal conditions.
**Option C:** Glucose can be utilized by cardiac muscle cells, especially during periods of high glucose availability or when fatty acid oxidation is impaired. Nevertheless, it is not the preferred energy source under normal conditions.
**Option D:** While all the options (A, B, and C) can be used by cardiac muscle cells under certain conditions, only fatty acids are the primary energy source under normal conditions.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the heart's energy requirements are met by a combination of fatty acid oxidation and glucose metabolism, with a shift towards glucose utilization during periods of high glucose availability or when fatty acid oxidation is impaired.

**✓ Correct Answer: A. Fatty acids**