**Core Concept**
Transvaginal ultrasound (TVUS) is the primary imaging modality used for diagnosing ectopic pregnancies due to its high sensitivity and specificity in detecting early gestational sacs and adnexal masses. TVUS allows for precise measurement of the adnexal mass and its proximity to the ovary and uterine corpus.

**Why the Correct Answer is Right**
TVUS is preferred over other imaging modalities like abdominal ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) due to its higher spatial resolution and ability to detect smaller adnexal masses. Additionally, TVUS can differentiate between an intrauterine pregnancy and an ectopic pregnancy, which is critical for guiding management decisions. The sensitivity of TVUS for detecting ectopic pregnancies is high, especially in the early stages of pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Abdominal ultrasound is less sensitive than TVUS for detecting early gestational sacs and adnexal masses, making it a less preferred choice for diagnosing ectopic pregnancies.
**Option B:** CT scans are not typically used for diagnosing ectopic pregnancies due to the risk of radiation exposure to the fetus and the fact that it is not as sensitive as TVUS.
**Option C:** MRI is not the first-line imaging modality for ectopic pregnancies due to its higher cost and limited availability compared to TVUS.

**Clinical Pearl / High-Yield Fact**
When using TVUS to diagnose an ectopic pregnancy, it is essential to look for the "ring of fire" sign, which is a hyperechoic ring surrounding the adnexal mass, indicating a ruptured ectopic pregnancy.

**Correct Answer: D. Transvaginal ultrasound (TVUS)**