**Core Concept**
The lactate dehydrogenase (LDH) enzyme is a tetrameric protein composed of multiple isozymes, each with a specific tissue distribution and functional role. LDH isozymes are classified into five main types (LD-1 to LD-5) based on their electrophoretic mobility, with each type consisting of varying combinations of M (muscle) and H (heart) subunits.

**Why the Correct Answer is Right**
The predominant isozyme of LDH in cardiac muscle is LD-1, which is composed of four H subunits (H4). This isozyme is specifically expressed in the heart and red blood cells, and its presence is often used as a biomarker for cardiac damage or myocardial infarction. The H subunits of LD-1 are more efficient at catalyzing the conversion of pyruvate to lactate, which is essential for the energy metabolism of cardiac muscle cells.

**Why Each Wrong Option is Incorrect**
**Option B:** LD-2 is composed of two H and two M subunits (H2M2), and is predominantly found in the lungs and kidneys. It is not the predominant isozyme in cardiac muscle.
**Option C:** LD-3 is composed of one H and three M subunits (H1M3), and is predominantly found in the liver and skeletal muscle. It is not the predominant isozyme in cardiac muscle.
**Option D:** LD-5 is composed of four M subunits (M4), and is predominantly found in the liver, skeletal muscle, and other tissues. It is not the predominant isozyme in cardiac muscle.

**Clinical Pearl / High-Yield Fact**
When interpreting LDH levels in a patient with suspected myocardial infarction, it is essential to note that LD-1 is the predominant isozyme in cardiac muscle. Elevated LD-1 levels can indicate cardiac damage, but it is also essential to consider the total LDH activity and other biomarkers, such as troponins, to confirm the diagnosis.

**✓ Correct Answer: A. LD-1**