**Core Concept:** Ultrasonography (USG) is a non-invasive imaging modality that utilizes high-frequency sound waves to create images of internal structures. In the context of congenital malformations, USG can be used to detect structural abnormalities in the developing fetus during pregnancy.

**Why the Correct Answer is Right:** The correct answer, **C. Fetus in Utero (FIO)**, refers to the earliest stage at which congenital malformations can be detected using ultrasound. This is because ultrasound waves can penetrate through bones and soft tissues, allowing for visualization of fetal structures. Detecting congenital malformations in the fetus in utero (FIO) provides valuable information for both the mother and the healthcare provider, enabling early intervention, management planning, and counseling.

**Why Each Wrong Option is Incorrect:**

A. **End of 1st trimester (12-14 weeks)**: While ultrasound may be performed during this stage, it is not the earliest time point for detecting congenital malformations. Fetal organs are not fully developed during the first trimester, making it challenging to identify structural abnormalities accurately.

B. **End of 2nd trimester (20-24 weeks)**: While ultrasound is more feasible during this stage, detecting malformations at this time point still presents limitations. Fetal organs may have developed, but they may not be fully formed, leading to inaccurate detection or misinterpretation of abnormalities.

D. **End of 3rd trimester (30-36 weeks)**: Similar to options A and B, detecting malformations in the 3rd trimester also has limitations. Some organs may be fully developed, but fetal growth and organ maturation may not be conclusive for accurate malformation diagnosis.

**Clinical Pearl:** Ultrasonography is an essential tool in obstetrics for monitoring fetal wellbeing, gestational age, and detecting complications like growth restriction or polyhydramnios. However, the most accurate detection of congenital malformations is during the **FETUS IN UTERO (FIO)** stage, as fetal organs are more developed and easier to visualize, allowing for more reliable diagnosis.