**Core Concept**
Granulosa cell tumors are a type of sex cord-stromal tumor that arises from the ovarian stroma, producing estrogen. They are a common cause of postmenopausal bleeding and are often diagnosed in women over the age of 50. The distinguishing feature of granulosa cell tumors is their production of specific proteins and enzymes that can be detected through immunohistochemical staining.

**Why the Correct Answer is Right**
Granulosa cell tumors are known to produce inhibin, which is a protein that inhibits the production of follicle-stimulating hormone (FSH) by the pituitary gland. Inhibin is produced by the granulosa cells of the ovary and is often used as a marker to diagnose granulosa cell tumors. The production of inhibin by granulosa cell tumors can be detected through immunohistochemical staining, which is a key feature that distinguishes them from other types of tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because alpha-inhibin is not a specific marker for granulosa cell tumors. While inhibin is produced by granulosa cells, alpha-inhibin is not a specific marker for this type of tumor.

**Option B:** Incorrect because beta-hCG is a marker for choriocarcinoma, a type of gestational trophoblastic tumor, not granulosa cell tumors.

**Option C:** Incorrect because PAX8 is a marker for a variety of tumors, including ovarian tumors, but it is not specific for granulosa cell tumors.

**Option D:** Incorrect because WT1 is a marker for yolk sac tumors, a type of germ cell tumor, not granulosa cell tumors.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that granulosa cell tumors are often estrogen-producing, leading to symptoms such as postmenopausal bleeding and endometrial hyperplasia. A high index of suspicion for granulosa cell tumors is necessary in women presenting with postmenopausal bleeding, especially if the tumor is large and multicystic.

**Correct Answer:** D.