What external factors can negatively impact embryonic development?

The correct response indicates that teratogens are external factors that can negatively impact embryonic development. Teratogens are substances or environmental factors that cause malformation of an embryo or fetus during pregnancy. Examples of teratogens include certain medications, drugs, chemicals, infectious agents, and even some maternal lifestyle choices such as alcohol consumption and smoking. When a developing embryo is exposed to these teratogens, the risk of structural anomalies or functional deficits increases significantly, leading to various birth defects.

Genetics and hormonal changes are essential components of development but are not classified as external factors that can disrupt embryonic development in the same way. Genetic factors are inherent to the individual and can result in congenital conditions if there are inherited anomalies, but they are not considered external disruptions. Hormonal changes, while they can influence certain development aspects, mainly play an internal regulatory role within the body rather than introducing external risks to the embryo's development.

Consequently, the focus on teratogens captures the essence of external influences that can adversely affect the developing embryo, making it the most appropriate answer to the question posed.