**Core Concept**
In pregnancy, especially during the third trimester, there are significant changes in maternal physiology, including renal function and electrolyte balance. The presence of certain substances in urine can serve as indicators of these changes.

**Why the Correct Answer is Right**
Human chorionic gonadotropin (hCG) is a hormone produced by the placenta during pregnancy. It plays a crucial role in maintaining the corpus luteum and supporting embryonic development. In the third trimester, hCG levels can be detected in urine, which can be used for pregnancy testing and monitoring. The presence of hCG in urine is a result of the placental production and its subsequent excretion by the kidneys.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not a relevant substance in urine during pregnancy.
* **Option B:** Although estriol is a hormone produced during pregnancy, it is not typically present in urine in significant amounts.
* **Option C:** This is not a substance that is commonly associated with pregnancy or found in urine.

**Clinical Pearl / High-Yield Fact**
hCG can be detected in urine as early as 6-8 days after conception, and its levels can be used to monitor pregnancy health and detect potential complications.

**Correct Answer:** C. Human chorionic gonadotropin (hCG)