## **Core Concept**
The question pertains to the reproductive physiology, specifically to the development and reserve of ovarian follicles in females. Primordial follicles represent the earliest stage of follicular development in the ovaries. They are formed during fetal development and consist of an oocyte surrounded by a layer of flat granulosa cells.

## **Why the Correct Answer is Right**
The correct answer, **D. 1-2 million**, reflects the estimated number of primordial follicles present in the ovaries of a female at birth. These follicles are formed during fetal development and serve as the resting pool of follicles, from which a subset is activated to grow and mature during each menstrual cycle. The number of primordial follicles peaks around 20 weeks of gestation, with estimates ranging from 1 to 2 million, and then gradually declines due to atresia.

## **Why Each Wrong Option is Incorrect**
- **Option A: 100-200 thousand** - This number is significantly lower than the estimated number of primordial follicles at birth. It more closely aligns with the number of follicles that remain at the onset of puberty or in early adulthood.
- **Option B: 500 thousand** - While closer, this still underestimates the actual number of primordial follicles present at birth.
- **Option C: 5-10 million** - This overestimates the number; the actual peak number of primordial follicles during fetal development might approach this range, but by birth, the number is estimated to be around 1-2 million.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the number of primordial follicles declines significantly from the peak during fetal development to birth and continues to decrease throughout a woman's life. By the time a woman reaches menopause, the number of primordial follicles has decreased to nearly zero. This decline is a natural process and is influenced by factors such as age, genetics, and environmental exposures.

## **Correct Answer: D. 1-2 million**