**Core Concept**
The clinical presentation of rapid onset hirsutism with a pelvic mass in a young woman is suggestive of an androgen-secreting tumor. This condition is often caused by an ovarian or adrenal tumor that produces excessive amounts of androgens, leading to virilization.

**Why the Correct Answer is Right**
The likely cause of this presentation is an ovarian tumor, specifically a granulosa cell tumor or a theca cell tumor. These tumors are known to produce estrogen and androgens, leading to hirsutism and virilization. The rapid onset of symptoms suggests a neoplastic process, and the presence of a pelvic mass supports this diagnosis. The tumor may also secrete human chorionic gonadotropin (hCG), which can stimulate the production of androgens in the tumor.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a thyroid disorder would not typically present with a pelvic mass or rapid onset hirsutism.
**Option B:** This option is incorrect because a pituitary tumor would not typically cause a pelvic mass or androgen excess, as it would primarily affect gonadotropin secretion.
**Option C:** This option is incorrect because polycystic ovary syndrome (PCOS) would typically present with chronic hirsutism and ovulatory dysfunction, rather than a rapid onset of symptoms.

**Clinical Pearl / High-Yield Fact**
A key feature of androgen-secreting tumors is the production of hCG, which can stimulate the production of androgens in the tumor. This is an important consideration in the diagnosis and management of these tumors.

**Correct Answer:** C. A theca cell tumor or granulosa cell tumor of the ovary.