**Core Concept**
Plasma anion gap (PAG) is a calculated value that represents the difference between the measured cations and anions in the blood. It is a crucial diagnostic tool used to identify the presence of unmeasured anions, which can be indicative of various metabolic disorders, including diabetic ketoacidosis, lactic acidosis, and renal failure.

**Why the Correct Answer is Right**
The plasma anion gap is calculated by subtracting the sum of chloride (Cl-) and bicarbonate (HCO3-) concentrations from the sum of sodium (Na+), potassium (K+), and calcium (Ca2+) concentrations. An increase in the plasma anion gap indicates the presence of unmeasured anions, such as lactate, ketones, or sulfates, which are not accounted for in the calculation. This can occur in conditions of increased metabolic acidosis, where the body produces more acidic substances than it can handle.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the plasma anion gap is not directly related to the respiratory system. While respiratory acidosis can lead to an increase in PaCO2, it does not directly affect the plasma anion gap.
**Option B:** This option is incorrect because the plasma anion gap is not a measure of kidney function. Although kidney disease can lead to changes in electrolyte balance, it does not directly affect the plasma anion gap.
**Option C:** This option is incorrect because the plasma anion gap is not a measure of blood pH. While pH can be affected by changes in the plasma anion gap, the two are not directly related.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the common causes of increased plasma anion gap is "LKS": Lactic acidosis, Ketoacidosis, and Sulfates (such as those found in aspirin poisoning).

**Correct Answer:** D.