**Core Concept**
Normal pregnancy can be earliest detected by transvaginal ultrasound (TVS) when the beta-human chorionic gonadotropin (β-hCG) levels are sufficiently high to visualize the gestational sac. This occurs due to the increased sensitivity and resolution of TVS, allowing for the detection of early embryonic structures.

**Why the Correct Answer is Right**
The correct answer is 1000 IU/mL because β-hCG levels typically rise exponentially during early pregnancy. At around 1000 IU/mL, the gestational sac becomes visible on TVS, indicating a normal intrauterine pregnancy. This is often referred to as the "empty follicle sign" or the "yolk sac sign," where the gestational sac is seen without the presence of an embryo. The high sensitivity of TVS allows for the detection of the gestational sac even in the absence of a visible embryo.

**Why Each Wrong Option is Incorrect**
**Option A:** 500 IU/mL is too low for the detection of the gestational sac on TVS, as the β-hCG levels may not be sufficient to visualize the embryo or the gestational sac.

**Option C:** 1500 IU/mL is still a reasonable threshold for the detection of the gestational sac, but 1000 IU/mL is generally considered the earliest time point for normal pregnancy detection by TVS.

**Option D:** 2000 IU/mL is higher than the threshold for the detection of the gestational sac, but it is not the earliest time point for normal pregnancy detection.

**Clinical Pearl / High-Yield Fact**
It is essential to note that TVS sensitivity and specificity increase with the level of β-hCG, allowing for earlier detection of normal pregnancies. However, it is crucial to correlate TVS findings with clinical history and β-hCG levels to avoid misinterpretation of findings.

**✓ Correct Answer: B. 1000 IU/mL**