**Core Concept**
Ectopic pregnancy is a life-threatening condition where the fertilized ovum implants outside the uterine cavity, most commonly in the fallopian tube. Accurate diagnosis is crucial to prevent maternal mortality. The diagnosis involves a combination of clinical evaluation, imaging studies, and laboratory tests.

**Why the Correct Answer is Right**
The diagnosis of ectopic pregnancy involves a thorough clinical evaluation, including a detailed medical history and physical examination. **Transvaginal ultrasound (TVUS)** is a crucial tool in diagnosing ectopic pregnancy, as it allows for the visualization of the adnexal mass and the uterine cavity. **Serial beta-hCG levels** can help identify a rising or plateauing level, which may indicate an ectopic pregnancy. **Pelvic ultrasound** is also useful in identifying the location and size of the adnexal mass.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **hysterosalpingography (HSG)** is not a primary diagnostic tool for ectopic pregnancy. Although HSG can demonstrate tubal patency, it is not as sensitive or specific as TVUS for diagnosing ectopic pregnancy.

**Option B:** This option is incorrect because **computed tomography (CT) scan** is not typically used as a first-line diagnostic tool for ectopic pregnancy. However, CT scan may be used in cases where ultrasound findings are inconclusive or when there is a high suspicion of ectopic pregnancy.

**Option C:** This option is incorrect because **magnetic resonance imaging (MRI)** is not typically used in the diagnosis of ectopic pregnancy. However, MRI may be used in cases where there is a need for further evaluation of the adnexal mass or when there are concerns for ovarian torsion.

**Clinical Pearl / High-Yield Fact**
In cases of suspected ectopic pregnancy, it is essential to remember the **"6-8-10 rule"**: a beta-hCG level of 6,000-8,000 IU/L or more, with a plateau or downward trend, in conjunction with an adnexal mass on TVUS, is highly suggestive of an ectopic pregnancy.

**Correct Answer: A. Hysterosalpingography (HSG) is not a primary diagnostic tool for ectopic pregnancy.**