**Core Concept**
The Twin-Peak sign is a sonographic marker observed in the first trimester of pregnancy, representing a significant finding in obstetric ultrasonography. It is related to the presence of a specific anatomical structure in the embryo.

**Why the Correct Answer is Right**
The Twin-Peak sign is indicative of an intrauterine pregnancy with a yolk sac, which is a normal structure present in early embryonic development. The yolk sac is a fluid-filled structure that provides nutrients to the embryo before the placenta develops. The Twin-Peak sign is observed when two peaks are seen on the ultrasound, corresponding to the yolk sac and the embryonic pole. This sign is considered a marker of a normal intrauterine pregnancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the Twin-Peak sign is not a marker for ectopic pregnancy, although an empty uterine cavity with no yolk sac or embryonic pole may suggest an ectopic pregnancy.
* **Option B:** Incorrect because the Double Decidua sign is a different sonographic marker used to diagnose ectopic pregnancy, not related to the Twin-Peak sign.
* **Option C:** Incorrect because the lambda sign is another sonographic marker used to diagnose twin pregnancies, not related to the Twin-Peak sign.

**Clinical Pearl / High-Yield Fact**
The Twin-Peak sign is a reassuring finding in early pregnancy, indicating a normal intrauterine pregnancy with a yolk sac. However, it is essential to evaluate the entire ultrasound image, including the presence of an embryonic pole and cardiac activity, to confirm a viable pregnancy.

**Correct Answer:** D.