**Core Concept**
During embryonic development, the formation of lanugo hair is a key process that contributes to the formation of the skin and its appendages. Lanugo hair is a type of fine, soft hair that covers the fetus's body, particularly during the second trimester of pregnancy.

**Why the Correct Answer is Right**
The appearance of lanugo hair is a result of the differentiation of hair follicles from the ectodermal layer of the embryo. This process is triggered by the interaction of various growth factors and hormones, including epidermal growth factor (EGF) and fibroblast growth factor (FGF). As the hair follicles mature, they produce lanugo hair, which helps to protect the fetus from the external environment and also aids in the regulation of body temperature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as lanugo hair appears during embryonic development and not in the postnatal period.
**Option B:** This option is incorrect as lanugo hair is not associated with the onset of puberty.
**Option C:** This option is incorrect as lanugo hair is not a characteristic feature of Down syndrome.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the presence of lanugo hair on a newborn can be a sign of intrauterine growth restriction (IUGR), where the fetus has not received adequate nutrients and oxygen for proper growth and development.

**Correct Answer: D. Third trimester. Lanugo hair appears at around 16-20 weeks of gestation and is usually shed by the time of birth.