**Core Concept**
The (A-a)O2 gradient, also known as the alveolar-arterial oxygen gradient, is a measure of the difference between the alveolar oxygen tension and the arterial oxygen tension. This gradient is an important indicator of the efficiency of gas exchange in the lungs.

**Why the Correct Answer is Right**
A normal (A-a)O2 gradient is typically less than 8-10 mmHg. This is because under normal conditions, the alveolar oxygen tension is very close to the arterial oxygen tension. An increased (A-a)O2 gradient suggests impaired gas exchange, which can be due to various conditions such as chronic obstructive pulmonary disease (COPD), pneumonia, or pulmonary embolism. The mechanism involves impaired diffusion of oxygen from the alveoli to the blood, often due to inflammation, edema, or structural damage to the lung parenchyma.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a normal (A-a)O2 gradient is typically less than 8-10 mmHg, not between 10-20 mmHg.
* **Option B:** This option is incorrect because a (A-a)O2 gradient greater than 35 mmHg is considered abnormal and suggests severe impairment of gas exchange.
* **Option C:** This option is incorrect because the (A-a)O2 gradient is not affected by the partial pressure of carbon dioxide (pCO2) in the blood.

**Clinical Pearl / High-Yield Fact**
A high (A-a)O2 gradient is a red flag for underlying pulmonary disease and should prompt further investigation, such as chest imaging or pulmonary function tests.

**Correct Answer:** C.