**Core Concept**
Human chorionic gonadotropin (HCG) is a hormone produced during pregnancy, playing a crucial role in maintaining the corpus luteum and supporting embryonic development. It is a glycoprotein hormone secreted by the placenta, primarily by the syncytiotrophoblasts.

**Why the Correct Answer is Right**
HCG is secreted by the syncytiotrophoblasts, which are the multinucleated cells formed by the fusion of cytotrophoblasts and other trophoblastic cells. The syncytiotrophoblasts are responsible for producing and secreting HCG, which then stimulates the corpus luteum to continue producing progesterone, essential for maintaining pregnancy. This hormone is crucial for the early stages of pregnancy, supporting the development of the embryo and maintaining the corpus luteum until the placenta takes over this function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option states that HCG is secreted by cytotrophoblasts, which is incorrect as HCG is primarily secreted by the syncytiotrophoblasts. Cytotrophoblasts are the precursor cells that fuse to form syncytiotrophoblasts.
**Option B:** Although HCG shares some structural similarity with luteinizing hormone (LH), it does not act on the same receptor as LH. HCG acts on the same receptor as LH, but it has a different mechanism of action and is not interchangeable with LH.
**Option C:** HCG has both luteotrophic and chorionic actions. The luteotrophic action of HCG maintains the corpus luteum, while its chorionic action supports embryonic development.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that HCG is a crucial hormone in early pregnancy, and its levels are often used as a marker for embryonic development and placental function. Abnormal HCG levels can indicate potential issues with pregnancy, such as ectopic pregnancy or chromosomal abnormalities.

**✓ Correct Answer: A. It is secreted by cytotrophoblasts**