**Core Concept:** Ectopic pregnancy is a type of pregnancy where the fertilized egg implants outside the uterus, most commonly in the fallopian tube. This can lead to life-threatening complications if left untreated. Transvaginal ultrasound (USG) is a valuable diagnostic tool for ectopic pregnancy.

**Why the Correct Answer is Right:** In an ectopic pregnancy, the characteristic finding on transvaginal ultrasound is the presence of a "ring sign". This sign represents the characteristic appearance of a fallopian tube containing a gestational sac and an embryo, both of which are surrounded by a mass of bleeding tissue that appears as a ring-like structure.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** The absence of a gestational sac and embryo in the uterus does not specifically point towards ectopic pregnancy. This could be present in intrauterine pregnancies as well.

B. **Option B:** The absence of a gestational sac and embryo in the uterus is also not specific to ectopic pregnancy. It could be seen in early intrauterine pregnancies.

C. **Option C:** The absence of a uterine endometrial signal on color Doppler is not specific to ectopic pregnancy. It could be seen in conditions like corpus luteal cyst or endometriosis.

D. **Option D:** The absence of a gestational sac and embryo in the uterus is not specific to ectopic pregnancy. It could be seen in early intrauterine pregnancies or miscarriages.

**Clinical Pearl:** The "ring sign" on transvaginal ultrasound is a crucial clue for diagnosing an ectopic pregnancy, especially when combined with other findings like absence of a gestational sac in the uterus and absence of endometrial signal on color Doppler. Timely diagnosis is essential to prevent life-threatening complications like rupture and haemoperitoneum.

**Correct Answer:** C. Absence of a gestational sac and embryo in the uterus (uterine) + absence of endometrial signal on color Doppler.

The correct answer is C because it encompasses the key findings on ultrasound for diagnosing ectopic pregnancy: absence of a gestational sac within the uterus (uterine) and absence of an endometrial signal on color Doppler. These findings are indicative of the implantation site outside the uterus, which is a hallmark of ectopic pregnancy. The other options are not specific enough to confirm ectopic pregnancy.