Okay, let's tackle this question about diagnosing ectopic pregnancy. The options are HCG, Transvaginal USG, AFP, and Gravindex. The question is asking for the modern diagnostic aid.

First, I need to recall what each of these tests does. HCG is a hormone used to confirm pregnancy, but it's not specific for ectopic. Transvaginal ultrasound (USG) is a common imaging method used in early pregnancy to visualize the gestational sac. AFP is alpha-fetoprotein, which is usually checked for neural tube defects, not ectopic. Gravindex is an old method that uses HCG levels and other factors, but I think it's outdated now.

The core concept here is the modern method for diagnosing ectopic pregnancy. Transvaginal USG is definitely the standard. It allows visualization of the uterine cavity and can detect an ectopic sac if the intrauterine pregnancy isn't visible. HCG levels are used in conjunction with USG, but alone they can't confirm ectopic. AFP and Gravindex are not used for this purpose anymore. So the correct answer should be B.

**Core Concept**
Ectopic pregnancy diagnosis relies on imaging to visualize the gestational sac and serum β-hCG levels. Transvaginal ultrasonography is the gold standard for confirming intrauterine gestation and detecting ectopic pregnancies due to its high resolution in early pregnancy.

**Why the Correct Answer is Right**
Transvaginal ultrasonography (USG) provides high-resolution imaging of the uterus, fallopian tubes, and adnexa. In a viable intrauterine pregnancy, the gestational sac should be visible by 35 days post-LMP. If β-hCG exceeds 6,500 mIU/mL and no intrauterine sac is seen, ectopic pregnancy is strongly suspected. USG can also identify adnexal masses, free fluid (suggesting hemorrhage), or fetal cardiac activity, which are critical for diagnosis and risk stratification.

**Why Each Wrong Option is Incorrect**
**Option A:** β-hCG confirms pregnancy but cannot distinguish ectopic from intrauterine. Serial measurements guide timing for USG but lack diagnostic specificity.
**Option C:** Alpha-fetoprotein (AFP) is used in neural tube defect screening and tumor markers, unrelated to ectopic pregnancy.
**Option D:** Gravindex (a formula using β-hCG and progesterone levels) is obsolete; modern protocols use USG and β-hCG kinetics instead.

**Clinical Pearl / High-Yield Fact**
Remember the "discriminatory zone" rule: If β-hCG > 6,500 mIU/mL and no intrauterine sac is seen on transvaginal USG, ectopic pregnancy is likely. Always correlate imaging with serum β-hCG levels and clinical findings like abdominal pain or vaginal bleeding.

**Correct Answer: B. Transvaginal USG**

✓ Correct Answer: D. ab