**Core Concept:**
The anion gap is a measure of the difference between the concentration of anions (negatively charged ions) and cations (positively charged ions) in the blood. In a healthy individual, the anion gap is typically influenced by the levels of non-anion gap components, such as lactate, chloride, and bicarbonate, and the anion gap components, such as albumin and glucose. The anion gap is calculated as follows: (Na+ + K+) - (Cl- + HCO3- + Glu + Lac).

**Why the Correct Answer is Right:**
In a healthy individual, the anion gap is typically around 8-12. This range is influenced by the normal concentrations of non-anion gap components (e.g., lactate, chloride, and bicarbonate) and anion gap components (e.g., albumin and glucose). The normal concentrations of these components contribute to a stable and relatively small anion gap, ensuring proper cellular function, acid-base balance, and overall homeostasis.

**Why Each Wrong Option is Incorrect:**
Option A: An anion gap of 0 indicates the absence of non-anion gap components, which is highly unlikely in a healthy individual.
Option B: An anion gap of 20 is extremely high and could indicate the presence of excess anions, such as in conditions like severe acidosis or severe dehydration.
Option C: An anion gap of 4 is much lower than the typical range and could suggest inappropriate levels of non-anion gap components or a lack of anion gap components.
Option D: An anion gap of 12 is higher than the typical range in a healthy individual but is still within the normal range for those with acidosis or other conditions that affect the anion gap, such as severe dehydration.

**Clinical Pearl:**
The interpretation of the anion gap requires considering the normal ranges of non-anion gap and anion gap components, as well as the clinical context of the patient's condition. A widened anion gap may indicate a need for further evaluation, such as in cases of severe acidosis, dehydration, or other conditions involving abnormal concentrations of non-anion gap components or anion gap components.

**Correct Answer:**
The correct answer, D, is within the normal range for an individual with acidosis or other conditions affecting the anion gap, such as severe dehydration. However, it is still higher than the typical range in a healthy individual. Understanding the anion gap and its components is essential for proper interpretation of acid-base balance and overall clinical assessment.