**Core Concept**
Lactate dehydrogenase (LDH) is an enzyme that catalyzes the conversion of lactate to pyruvate, playing a crucial role in energy metabolism. LDH exists in various isoforms, each with distinct tissue distributions and clinical implications.

**Why the Correct Answer is Right**
LDH has five isoenzymes, denoted as LD1 to LD5, which are formed by different combinations of the M and H subunits. These isoenzymes have varying levels of activity in different tissues, such as the heart (LD1), liver (LD2), skeletal muscle (LD3), lungs (LD4), and kidneys (LD5). The distribution of LDH isoenzymes can be useful in diagnosing tissue damage or disease, as certain patterns of isoenzyme elevation can indicate specific conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because LDH isoenzymes are not limited to the number specified here.
**Option B:** This option is incorrect because LDH isoenzymes are not solely based on the presence or absence of a single subunit.
**Option C:** This option is incorrect because it does not accurately reflect the number of LDH isoenzymes.

**Clinical Pearl / High-Yield Fact**
Recall that the ratio of LD1 to LD2 isoenzymes can be useful in diagnosing myocardial infarction, as an elevated LD1/LD2 ratio is often seen in cardiac tissue damage.

**Correct Answer:** C. 5. LDH has five isoenzymes.