**Core Concept**
Precocious puberty is defined as the onset of secondary sexual characteristics before the age of 8 in girls and 9 in boys. The diagnosis hinges on timing, with the most common cause being constitutional (genetic) in origin, especially in cases without underlying pathology.

**Why the Correct Answer is Right**
The standard definition for precocious puberty in girls is the appearance of secondary sexual characteristics before age 8. While some sources may cite 6 years as a threshold, the widely accepted cutoff is 8 years. Option B states "before age 6 years," which is **not** the standard definition and is too early. Most cases (especially idiopathic) are constitutional, meaning they represent a normal variation in growth patterns. Thus, B is the exception and therefore the correct choice.

**Why Each Wrong Option is Incorrect**
Option A: Secondary sexual characteristics before age 8 in girls is the accepted definition of precocious puberty, so this is true.
Option C: Menstruation before age 10 in girls is considered precocious, as it occurs earlier than the typical age of 12–13, so this is true.
Option D: The most common cause of precocious puberty is constitutional, especially in children with normal anatomy and no structural disease, so this is true.

**Clinical Pearl / High-Yield Fact**
Remember: **Precocious puberty = secondary sexual characteristics before age 8 in girls**. Before 6 years, it's more likely to be a rare condition like central nervous system or hypothalamic disorders, not typical constitutional precocity.

✓ Correct Answer: B. Secondary sexual characters before the age of 6 years