## **Core Concept**
The embryonic period is a critical phase in human development, spanning from fertilization to the end of the eighth week of gestation. During this period, the major organs and body systems begin to form, and the embryo is highly susceptible to teratogenic insults. This period is characterized by rapid growth and differentiation of cells.

## **Why the Correct Answer is Right**
The embryonic period starts from fertilization, which typically occurs in the fallopian tube, and lasts until the end of the eighth week of gestation. This period is crucial for organogenesis, where the three primary germ layers (ectoderm, mesoderm, and endoderm) give rise to the major organs and body systems. The correct answer, therefore, corresponds to the timeframe from fertilization to the end of the eighth week.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately represent the duration of the embryonic period.
- **Option B:** This option is incorrect as it either starts or ends at an incorrect point in gestation.
- **Option C:** This option is incorrect because it does not correctly identify the start and end points of the embryonic period.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the embryonic period is from fertilization (week 1) to the end of week 8. After week 8, the term "fetus" is used, and this period is known as the fetal period. During the embryonic period, the embryo is particularly vulnerable to teratogens, which can cause congenital anomalies.

## **Correct Answer:** D. From fertilization to end of 8th week.