**Core Concept**
Premature thelarche and adrenarche followed by vaginal bleeding in a prepubertal girl can be indicative of central precocious puberty (CPP). CPP is a condition where the hypothalamic-pituitary-gonadal axis is activated prematurely, leading to early sexual development. The most common cause of CPP in girls is idiopathic, meaning the underlying cause is unknown.

**Why the Correct Answer is Right**
Idiopathic CPP is diagnosed when there is no identifiable cause for the early activation of the hypothalamic-pituitary-gonadal axis. In these cases, the hypothalamus secretes gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The increased levels of LH and FSH stimulate the ovaries to produce estrogen, leading to the characteristic symptoms of CPP, including premature thelarche, adrenarche, and vaginal bleeding. The exact cause of idiopathic CPP is unknown, but it is thought to be related to genetic or environmental factors.

**Why Each Wrong Option is Incorrect**
**Option B:** Gonadal tumors can cause precocious puberty, but they are a rare cause, especially in the absence of other symptoms such as abdominal masses or pain.

**Option C:** McCune-Albright syndrome is a genetic disorder that can cause CPP, but it is much less common than idiopathic CPP. It is characterized by café-au-lait skin spots, polyostotic fibrous dysplasia, and endocrine abnormalities, including CPP.

**Option D:** Hypothyroidism can cause delayed puberty, not precocious puberty. In hypothyroidism, the hypothalamic-pituitary-gonadal axis is not stimulated prematurely, but rather is suppressed, leading to delayed sexual development.

**Clinical Pearl / High-Yield Fact**
It's essential to note that CPP is a diagnosis of exclusion, and a thorough evaluation should be performed to rule out underlying causes of precocious puberty. A bone age assessment, hormonal evaluations, and imaging studies may be necessary to confirm the diagnosis of idiopathic CPP.

**✓ Correct Answer: A. Idiopathic**