**Core Concept**
Ectopic pregnancy occurs when a fertilized ovum implants outside the uterine cavity, most commonly in the fallopian tube. Early detection is crucial to prevent life-threatening complications. Transvaginal ultrasound is a valuable diagnostic tool for assessing the location of the gestational sac.

**Why the Correct Answer is Right**
Transvaginal ultrasound (TVUS) allows for high-resolution imaging of the pelvic organs, enabling the visualization of the fallopian tubes and the uterine cavity. This modality can detect an intrauterine gestational sac, which rules out an ectopic pregnancy. TVUS can also identify free fluid in the adnexal region, suggesting a possible ectopic pregnancy. The sensitivity and specificity of TVUS make it an essential tool in the diagnosis of ectopic pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Serial beta-hCG levels can help identify a failing intrauterine pregnancy but are not definitive in diagnosing an ectopic pregnancy. Rising beta-hCG levels in the absence of an intrauterine gestational sac may suggest an ectopic pregnancy, but this is not a diagnostic test.

**Option C:** Progesterone measurement is not a reliable diagnostic tool for ectopic pregnancy. Low progesterone levels may be seen in cases of ectopic pregnancy, but this is not specific and can be seen in other conditions as well.

**Option D:** Culdocentesis, or the aspiration of fluid from the posterior cul-de-sac, was a historical method used to diagnose ectopic pregnancy. However, it is invasive and has largely been replaced by TVUS due to its high sensitivity and specificity.

**Clinical Pearl / High-Yield Fact**
A "double decidual sac sign" seen on TVUS is a specific indicator of an intrauterine gestational sac, which rules out an ectopic pregnancy. This sign is characterized by a double-layered appearance of the decidua capsularis and decidua parietalis surrounding the gestational sac.

**✓ Correct Answer: B. Transvaginal USG**