**Question:** Best modality to diagnose unruptured tubal pregnancy is:

A. Transvaginal ultrasound
B. Serum beta-human chorionic gonadotropin (beta-hCG) assay
C. Computed tomography (CT) scan
D. Magnetic resonance imaging (MRI)

**Correct Answer: A (Transvaginal ultrasound)**

**Core Concept:**
Tubal pregnancies are ectopic pregnancies that occur within the fallopian tubes. Diagnosis of tubal pregnancies is crucial to avoid potential life-threatening complications like rupture, hemorrhage, and severe hemorrhagic shock. Early detection and appropriate management can prevent these complications and maintain maternal health.

**Why the Correct Answer is Right:**
Transvaginal ultrasound is the preferred modality for diagnosing tubal pregnancies due to its high resolution and ability to visualize the uterus and adnexa (ovaries and fallopian tubes) in detail. Transvaginal ultrasound can detect the presence of a gestational sac, fetal cardiac activity, and gestational age, which are essential for confirming the diagnosis.

**Why Each Wrong Option is Incorrect:**

**A. Serum beta-hCG assay (beta-human chorionic gonadotropin assay):**
While serum beta-hCG assay is helpful in monitoring the progress of treatment and evaluating the effectiveness of beta-hCG-based therapy, it is not sufficient as a standalone diagnostic tool. A rising beta-hCG level may indicate ectopic pregnancy, but its confirmation requires imaging for the location and assessment of fetal cardiac activity.

**B. Computed tomography (CT) scan:**
A CT scan is typically used for evaluating pelvic infections or trauma but is not the first-line choice for diagnosing tubal pregnancies. CT scans expose the patient to ionizing radiation, which is not recommended for pregnant patients.

**C. Magnetic resonance imaging (MRI):**
MRI is a valuable tool in evaluating soft tissue and brain disorders but is not the preferred modality for diagnosing tubal pregnancies due to its higher cost, longer examination time, and lack of direct imaging of the adnexa.

**Clinical Pearls:**

1. Transvaginal ultrasound is the gold standard for diagnosing tubal pregnancies, as it directly visualizes the gestational sac, fetal cardiac activity, and adnexal mass.
2. Beta-hCG levels can confirm an ectopic pregnancy and monitor treatment response, but they lack the spatial resolution to localize the site of ectopic pregnancy.