**Core Concept**
The earliest detection of pregnancy by ultrasound is based on the visualization of a gestational sac within the uterus, which is a fluid-filled structure that forms around the embryo. This occurs before the embryo is visible, and it is an essential step in confirming an intrauterine pregnancy.

**Why the Correct Answer is Right**
The gestational sac is the first visible structure in an early pregnancy, usually detectable by transvaginal ultrasound between 4.5 to 5 weeks of gestation. It is formed by the aggregation of trophoblastic cells and is surrounded by a thin membrane. The presence of a gestational sac is a critical finding in early pregnancy, as it confirms the location of the embryo within the uterus and helps to rule out ectopic pregnancy.

**Why Each Wrong Option is Incorrect**

**Option A:** FSH (Follicle Stimulating Hormone) is a hormone produced by the pituitary gland that regulates ovulation and follicular growth in the ovaries. While FSH levels may be elevated in early pregnancy, it is not a direct indicator of pregnancy and is not used for ultrasound detection.

**Option B:** Fetal node is not a recognized anatomical structure in early pregnancy. While the fetal heart may be visible by transvaginal ultrasound around 6-7 weeks of gestation, it is not the earliest detectable feature.

**Option D:** Fetal skeleton is not visible on ultrasound until around 10-12 weeks of gestation, making it an unlikely candidate for the earliest detection of pregnancy.

**Clinical Pearl / High-Yield Fact**
The earliest detection of a gestational sac by ultrasound is a critical finding in early pregnancy, and its absence may indicate a failed pregnancy or an ectopic pregnancy.

**✓ Correct Answer: A. Gestation sac**