Consider the following four events of development of fetus: UPSC 08; COMEDK 15 Development of external genitalia Appearance of scalp hair Centers of ossification in bones Formation of eyelashes and eye brows What is the order in which they appear from lower to higher gestation?

**Core Concept**
The question tests the understanding of fetal development milestones, specifically the timing of external genitalia development, appearance of scalp hair, centers of ossification in bones, and formation of eyelashes and eyebrows. These events are crucial in assessing fetal maturity and gestational age.

**Why the Correct Answer is Right**
The correct sequence of these events from lower to higher gestation age is related to the embryological development timeline. External genitalia development occurs early, around 12-14 weeks, as part of the sexual differentiation process. Centers of ossification in bones start appearing around the same time or slightly later. The appearance of scalp hair, eyelashes, and eyebrows happens later in gestation, with eyelashes and eyebrows forming around 20-22 weeks and scalp hair appearing around 24-28 weeks.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the known timeline of fetal development.
**Option B:** Similarly, this option misrepresents the sequence of developmental milestones.
**Option D:** This option also incorrectly orders the events of fetal development.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the development of external genitalia and the appearance of centers of ossification are early signs of fetal development, while the formation of hair, eyelashes, and eyebrows are later indicators of gestational age.

**Correct Answer:** C. Centers of ossification, external genitalia, eyelashes and eyebrows, scalp hair

✓ Correct Answer: D. 3, 1, 2, 4