**Core Concept**
Human chorionic gonadotropin (HCG) is a hormone produced during pregnancy that is essential for maintaining the corpus luteum and supporting embryonic development. It is a glycoprotein hormone that shares structural and functional similarities with luteinizing hormone (LH).

**Why the Correct Answer is Right**
HCG is secreted by the syncytiotrophoblast, a layer of multinucleated cells that form the outer layer of the blastocyst during early embryonic development. The syncytiotrophoblast produces HCG as a response to implantation of the embryo into the uterine lining, promoting the maintenance of the corpus luteum and preventing its regression. This ensures the continued production of progesterone, which is essential for sustaining embryonic development. The syncytiotrophoblast continues to produce HCG throughout pregnancy, and its levels can be used as a marker for fetal development and placental function.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because HCG is not produced by the fetal liver. The fetal liver does produce various hormones and enzymes, but HCG is not one of them.
* **Option B:** Incorrect because HCG is not produced by the placental decidua. While the decidua is an essential component of the placenta, it is not responsible for producing HCG.
* **Option D:** Incorrect because HCG is not produced by the corpus luteum itself. The corpus luteum produces progesterone in response to LH, but it does not produce HCG.

**Clinical Pearl / High-Yield Fact**
HCG is a critical marker for pregnancy and can be used to diagnose and monitor pregnancy complications, such as ectopic pregnancy and miscarriage. It is also used in the treatment of infertility and as a component of assisted reproductive technologies.

**Correct Answer: C. syncytiotrophoblast. HCG secreted from the syncytiotrophoblast.**