**Core Concept**
Hyperkalemia is a condition characterized by elevated potassium ion levels in the blood, which can lead to various cardiac and neuromuscular complications. The normal range for serum potassium is approximately 3.5-5.5 mmol/L, and any level above this range is considered hyperkalemia.

**Why the Correct Answer is Right**
The correct answer is that hyperkalemia means more than 5.5 mmol/L of potassium in the blood. This is because potassium ions play a crucial role in maintaining the resting membrane potential of cardiac cells, and elevated levels can lead to cardiac arrhythmias and even cardiac arrest. The threshold for hyperkalemia is generally considered to be above 5.5 mmol/L, although some sources may quote slightly different values.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hyperkalemia can occur with potassium levels above the normal range, but it is not defined by levels below 5.5 mmol/L.

**Option B:** This option is incorrect because hyperkalemia specifically refers to elevated potassium levels, not low levels.

**Option C:** This option is incorrect because hyperkalemia is not defined by a specific range, but rather by levels above the normal range.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hyperkalemia can be caused by a variety of factors, including potassium supplements, certain medications, and kidney disease. A classic exam trap is to confuse hyperkalemia with hypokalemia, which refers to low potassium levels.

**Correct Answer: D. 5.5**