**Core Concept**
The question requires an understanding of the physiological pathways of drug excretion and metabolism in the human body. In this case, we are specifically looking at the excretion of substances in saliva, which is an important aspect of pharmacology and toxicology.

**Why the Correct Answer is Right**
Substances that are excreted in saliva are typically lipophilic and can diffuse through the salivary gland cells. One such example is **methotrexate**, a chemotherapy agent that is excreted in saliva. This is due to its lipophilic nature, which allows it to cross the cell membrane and be secreted into the saliva. The salivary gland's role in excreting lipophilic substances is an important consideration in the management of patients receiving certain medications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a substance that is excreted in saliva. Without further information, it is difficult to determine why this option is incorrect.
**Option B:** This option is incorrect because it does not mention a specific substance that is excreted in saliva. There is no clear evidence to support the excretion of this substance in saliva.
**Option C:** This option is incorrect because it does not provide a valid example of a substance excreted in saliva. While it may be a medication, there is no clear evidence to support its excretion in saliva.

**Clinical Pearl / High-Yield Fact**
The salivary gland's role in excreting lipophilic substances is an important consideration in the management of patients receiving certain medications. This is particularly relevant in the context of chemotherapy agents, where the excretion of these substances in saliva can have significant implications for patient care.

**Correct Answer:** C. Methotrexate