**Core Concept**
Normal venous oxygen saturation is a reflection of the balance between oxygen delivery and consumption in tissues. It is an indicator of tissue oxygenation and is affected by factors such as cardiac output, hemoglobin concentration, and oxygen extraction ratio.

**Why the Correct Answer is Right**
The normal partial pressure of oxygen in venous blood is approximately 40 mmHg, reflecting the amount of oxygen remaining in the blood after it has passed through the tissues. This is a result of the oxygen extraction ratio, which is the percentage of oxygen extracted from the blood by the tissues. The oxygen extraction ratio is typically around 25-30% at rest, with the remaining oxygen being returned to the lungs via the venous system. The partial pressure of oxygen in venous blood is an important parameter in assessing tissue oxygenation and can be affected by various conditions such as shock, anemia, or respiratory disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it is not a commonly accepted value for normal venous oxygen partial pressure.
**Option B:** This option is incorrect as it is too high and would indicate an excessive amount of oxygen remaining in the blood.
**Option C:** This option is incorrect as it is too low and would indicate a significant decrease in tissue oxygenation.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that normal venous oxygen saturation is a dynamic parameter that can be affected by various factors, including changes in cardiac output, hemoglobin concentration, and oxygen extraction ratio. A decrease in venous oxygen saturation can indicate tissue hypoxia or inadequate oxygen delivery.

**Correct Answer: C. 40 mmHg**