**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 primarily synthesized by the syncytiotrophoblast, the outer layer of the trophoblast cells in the placenta.

**Why the Correct Answer is Right**
The correct answer is B because HCG is indeed synthesized by the syncytiotrophoblast, not the cytotrophoblast. The syncytiotrophoblast is a multinucleated cell layer that develops from the fusion of cytotrophoblast cells, and it is responsible for the production of HCG. This hormone is essential for maintaining the pregnancy by supporting the corpus luteum and regulating the production of progesterone.

**Why Each Wrong Option is Incorrect**
**Option A:** HCG is actually a glycoprotein, not a glycopeptide. Although the terms are often used interchangeably, glycoproteins are proteins that have carbohydrate molecules attached, whereas glycopeptides are peptides with carbohydrate molecules attached.

**Option C:** The doubling time of HCG is typically around 48-72 hours, not 2 days. This characteristic doubling time is a useful marker for diagnosing and monitoring pregnancy.

**Option D:** High levels of HCG are indeed seen in multiple pregnancies, such as twins or triplets. This is because there are more trophoblastic cells producing HCG, resulting in higher concentrations.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that HCG levels are typically elevated in multiple gestations and ectopic pregnancies, making them important markers for diagnosing these conditions.

**✓ Correct Answer: B. Synthesized by cytotrophoblast**