## **Core Concept**
The core concept being tested here involves the use of ultrasonography (USG) in fetal development assessment, specifically focusing on the external genitalia. The question aims to evaluate the understanding of fetal development milestones that can be detected via ultrasound.

## **Why the Correct Answer is Right**
The correct answer, , indicates that the external genitalia can be earliest diagnosed by USG by 16 weeks. During fetal development, the external genitalia begin to form early, but their differentiation and detailed visualization through ultrasound become possible around this gestational age. The development of external genitalia is a critical milestone that can be assessed via ultrasound, helping in the determination of fetal sex and detection of any abnormalities.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 10 weeks - This is too early for the detailed assessment of external genitalia through USG. Although some early signs may be visible, it's generally not the time for definitive diagnosis.
- **Option B:** 12 weeks - While advancements in ultrasound technology have improved early detection capabilities, 12 weeks might still be early for a definitive assessment of external genitalia.
- **Option D:** 20 weeks - This is later than necessary for the earliest diagnosis of external genitalia. By 20 weeks, the external genitalia are well-developed, but the question asks for the earliest diagnosis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the assessment of fetal genitalia via ultrasound is not only important for determining fetal sex but also for identifying potential congenital anomalies. The optimal time for such assessments can vary slightly based on ultrasound technology and the skill of the operator, but generally, around 16 weeks is considered a critical period for detailed fetal anatomy surveys, including the external genitalia.

## **Correct Answer:** . 16 weeks.