## **Core Concept**
The question tests knowledge of lactate dehydrogenase (LDH) isoenzymes and their distribution in different tissues. LDH is an enzyme found in nearly all living cells, and it exists in five isoenzyme forms, which are tetramers composed of different proportions of two types of subunits: H (heart) and M (muscle). These isoenzymes play a critical role in anaerobic glycolysis.

## **Why the Correct Answer is Right**
The correct answer, LDH-2, is not accurate; actually, **LDH-1** is the predominant isoenzyme in cardiac muscle. This isoenzyme is composed of four H subunits (H4). The heart predominantly uses aerobic metabolism but has a high capacity for anaerobic glycolysis, which LDH facilitates. LDH-1 has a high affinity for pyruvate and is most active in tissues with high demands for anaerobic metabolism, such as the heart.

## **Why Each Wrong Option is Incorrect**
- **Option A (LDH-1):** While LDH-1 is actually the correct isoenzyme predominantly found in cardiac muscle, let's assume it was listed as an option; its characteristics make it suitable for cardiac tissue.
- **Option B (LDH-2):** This is the predominant isoenzyme in the serum of healthy individuals but not in cardiac muscle. It's composed of 3H and 1M subunits (H3M).
- **Option C (LDH-3):** This isoenzyme is primarily found in the lungs and other tissues but not predominantly in cardiac muscle. It consists of 2H and 2M subunits (H2M2).
- **Option D (LDH-5):** This isoenzyme, composed of four M subunits (M4), is predominantly found in skeletal muscle and the liver, not cardiac muscle.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that LDH-1 is elevated in conditions like myocardial infarction. The ratio of LDH-1 to LDH-2 can be used to diagnose myocardial infarction, especially when troponins are not elevated.

## **Correct Answer:** . LDH-1