**Core Concept**
During starvation, the body's metabolic state undergoes significant changes to adapt to the lack of glucose as a primary energy source. The heart, being a high-energy-demanding organ, must find alternative fuels to maintain its function.

**Why the Correct Answer is Right**
The heart primarily utilizes fatty acids as its major metabolic fuel during starvation. This is due to the increased breakdown and release of stored triglycerides from adipose tissue, resulting in elevated levels of free fatty acids in the bloodstream. The heart then takes up these fatty acids and uses them for energy production through beta-oxidation, producing acetyl-CoA, which is then fed into the citric acid cycle to generate ATP. This shift towards fatty acid oxidation is mediated by the increased activity of hormone-sensitive lipase, which breaks down triglycerides, and the activation of carnitine palmitoyltransferase 1 (CPT1), which facilitates the transport of long-chain fatty acids into the mitochondria.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, although ketone bodies can be used by the heart as an energy source during prolonged fasting or starvation, they are not the primary fuel used by the heart.
**Option B:** This option is incorrect because, although lactate can be used by the heart as an energy source during intense exercise or ischemia, it is not the primary fuel used by the heart during starvation.
**Option C:** This option is incorrect because, although glucose can be used by the heart as an energy source, it is not the primary fuel used by the heart during starvation due to the lack of glucose availability.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the heart's preference for fatty acids as a fuel source during starvation is a critical adaptation that allows it to maintain its function despite the lack of glucose availability. This knowledge is crucial for understanding the pathophysiology of cardiac dysfunction in conditions such as diabetic ketoacidosis or prolonged fasting.

**Correct Answer:** C. Fatty acids