**Core Concept**
Ectopic pregnancy occurs when a fertilized egg implants outside the uterus, most commonly in the fallopian tube. Accurate diagnosis is crucial to prevent maternal morbidity and mortality. The best investigation should be highly sensitive and specific for ectopic pregnancy.

**Why the Correct Answer is Right**
The most accurate investigation for ectopic pregnancy is transvaginal ultrasound (TVUS). TVUS allows for high-resolution imaging of the adnexal region, enabling the detection of an ectopic pregnancy as early as 4-5 weeks of gestation. The use of Doppler flow studies can also help identify the presence of a yolk sac or embryonic cardiac activity, which can confirm an intrauterine pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Beta-hCG levels alone are not sufficient for diagnosis, as they can be elevated in both intrauterine and ectopic pregnancies. Beta-hCG levels may also be normal in cases of early ectopic pregnancy.

**Option B:** Pelvic CT scan is not the preferred initial imaging modality for suspected ectopic pregnancy due to radiation exposure and the risk of inducing a hemoperitoneum. Additionally, CT scans may not be as sensitive as TVUS for detecting small ectopic pregnancies.

**Option C:** Pelvic MRI is not typically used for the initial evaluation of suspected ectopic pregnancy due to its higher cost and limited availability compared to TVUS.

**Clinical Pearl / High-Yield Fact**
In cases of suspected ectopic pregnancy, a negative TVUS does not rule out the diagnosis, and serial beta-hCG levels and repeat ultrasound should be performed to confirm the diagnosis.

**Correct Answer: C. Pelvic CT scan.**