**Core Concept**
The alpha chain of human chorionic gonadotropin (hCG) shares structural similarity with other glycoprotein hormones, highlighting the conservation of hormone structure and function across different physiological roles. This similarity is crucial for the binding of these hormones to their respective receptors. The **alpha subunit** of hCG is nearly identical to the alpha subunits of **luteinizing hormone (LH)**, **follicle-stimulating hormone (FSH)**, and **thyroid-stimulating hormone (TSH)**.

**Why the Correct Answer is Right**
The alpha chain of hCG is identical to the alpha chains of LH, FSH, and TSH, as these hormones all share a common alpha subunit. This commonality allows for similar receptor binding characteristics, with the specificity of hormone action being determined by the **beta subunit** of each hormone. The beta subunit is unique to each hormone and confers the specific biological activity.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the alpha chain of hCG is indeed identical to that of LH.
**Option B:** Incorrect as the alpha chain of hCG is also identical to that of FSH.
**Option C:** Incorrect since the alpha chain of hCG shares identity with the alpha chain of TSH.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that while the alpha subunits of these glycoprotein hormones are highly conserved, the beta subunits are hormone-specific and determine the unique functions of each hormone. This distinction is crucial for understanding both the shared and distinct actions of these hormones in the body.

**Correct Answer:** D.