**Core Concept**
Calf compartment pressure increases due to muscle pump function during walking. The muscle pump generates a pressure gradient that helps to return blood from the lower extremities to the heart, but it can also lead to increased pressure within the calf compartments, particularly if the muscles are overworked or if there is a pre-existing condition such as deep vein thrombosis (DVT).

**Why the Correct Answer is Right**
The muscle pump function is essential for maintaining venous return and preventing blood stasis in the lower extremities. During walking, the calf muscles contract and relax in a rhythmic manner, creating a pressure gradient that helps to return blood from the lower extremities to the heart. This pressure gradient is generated by the increase in intramuscular pressure, which can rise to 30-40 mmHg during walking. The exact pressure at which calf compartment pressure rises during walking is not well-defined, but it is generally accepted to be in the range of 20-50 mmHg.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is too low and does not accurately reflect the pressure increase seen during walking.
**Option B:** This option is too high and may be associated with conditions such as DVT or compartment syndrome, which require immediate medical attention.
**Option C:** This option is not a valid measurement and does not accurately reflect the pressure increase seen during walking.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that increased calf compartment pressure can be a sign of underlying conditions such as DVT, compartment syndrome, or peripheral artery disease. A thorough history and physical examination, including assessment of calf compartment pressure, can help to identify these conditions and prevent long-term morbidity.

**Correct Answer: 30. 30 mmHg**