**Core Concept**
Decreased arterial PO2 in hypoxia is a critical indicator of oxygen deprivation in the body. This condition occurs when there is an imbalance between oxygen supply and demand, often due to impaired gas exchange in the lungs or increased oxygen consumption.

**Why the Correct Answer is Right**
In hypoxia, the partial pressure of oxygen (PO2) in arterial blood decreases. This occurs due to impaired diffusion of oxygen from the alveoli into the pulmonary capillaries. The primary cause of this impairment is the disruption of the normal gas exchange process in the lungs, often resulting from conditions such as chronic obstructive pulmonary disease (COPD), pneumonia, or pulmonary embolism. As a result, the oxygen-hemoglobin dissociation curve shifts to the left, making it more difficult for hemoglobin to release oxygen to the tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable, as this option is missing.
**Option B:** Not directly related to the decrease in arterial PO2 in hypoxia, as it refers to the increase in carbon dioxide levels.
**Option C:** Incorrect, as this option refers to the increase in partial pressure of carbon dioxide (PCO2) in the blood, which is a separate condition known as hypercapnia.

**Clinical Pearl / High-Yield Fact**
In severe hypoxia, the body's compensatory mechanisms may fail, leading to tissue hypoxia and potentially life-threatening conditions. It is essential for clinicians to quickly identify and address the underlying cause of hypoxia to prevent further complications.

**Correct Answer: C. Increase in carbon dioxide levels.