**Core Concept**
The question is testing the understanding of the relationship between liver function and the plasma levels of certain substances. In severe liver disease, the liver's ability to metabolize and clear these substances from the bloodstream is impaired, leading to elevated plasma levels. However, the question asks about substances that are likely to have lower plasma levels due to liver disease.

**Why the Correct Answer is Right**
The correct answer is likely to be a substance that is either synthesized or stored in the liver, or a substance that is normally metabolized or cleared by the liver. In severe liver disease, the liver's ability to synthesize and store these substances is impaired, leading to lower plasma levels. For example, the liver synthesizes clotting factors, such as prothrombin and fibrinogen, which are essential for blood coagulation. In liver disease, the synthesis of these clotting factors is impaired, leading to lower plasma levels.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because liver disease typically leads to elevated plasma levels of substances that are metabolized or cleared by the liver.
* **Option B:** This option is incorrect because liver disease typically leads to elevated plasma levels of substances that are synthesized or stored in the liver.
* **Option C:** This option is incorrect because liver disease typically leads to elevated plasma levels of substances that are metabolized or cleared by the liver.
* **Option D:** This option is incorrect because liver disease typically leads to elevated plasma levels of substances that are metabolized or cleared by the liver.

**Clinical Pearl / High-Yield Fact**
In severe liver disease, the synthesis of clotting factors is impaired, leading to an increased risk of bleeding. This is because the liver is unable to produce sufficient amounts of prothrombin and fibrinogen, which are essential for blood coagulation.

**Correct Answer:**
C. Prothrombin