**Core Concept**
The question tests the understanding of the effect of heparin on blood samples used for acid-base gas (ABG) analysis. Heparin is an anticoagulant commonly used in blood collection tubes to prevent clotting. However, it can also affect the results of certain laboratory tests.

**Why the Correct Answer is Right**
Heparin can bind to calcium ions, which are essential for the proper functioning of the electrode used in the pH meter to measure the pH of the blood sample. When heparin binds to calcium, it reduces the availability of free calcium ions, leading to a decrease in the pH measured by the electrode. This is because the electrode relies on the presence of calcium ions to accurately measure the pH. As a result, the pH value obtained from the blood sample may be artificially decreased. This effect is specific to the pH measurement and does not affect the measurement of pCO2, HCO3-, or pO2.

**Why Each Wrong Option is Incorrect**
**Option A:** The presence of heparin does not directly affect the measurement of pCO2, which is typically measured using a different technique, such as infrared spectroscopy.
**Option B:** Heparin does not directly affect the measurement of HCO3-, which is typically measured using a different technique, such as titration.
**Option D:** The presence of heparin does not affect the measurement of pO2, which is typically measured using a different technique, such as polarography.

**Clinical Pearl / High-Yield Fact**
When collecting blood samples for ABG analysis, it is essential to use the correct type of blood collection tube, such as a heparinized tube for blood gas analysis or a citrated tube for coagulation studies.

**✓ Correct Answer: C. pH**