**Core Concept:** Ectopic pregnancy is a type of pregnancy where the fertilized egg implants outside the uterus, usually in the fallopian tube. Diagnosis of ectopic pregnancy involves a combination of clinical presentation, laboratory tests, and imaging modalities.

**Why the Correct Answer is Right:**

The correct answer, **Option C (serum beta-human chorionic gonadotropin [β-hCG] level),** is right because it is a sensitive and specific test for detecting the presence of hCG in the blood. Although serum β-hCG is a crucial diagnostic tool, it does not help in excluding ectopic pregnancy.

**Why Each Wrong Option is Incorrect:**

Option A (transvaginal ultrasound) is incorrect because it can show an ectopic pregnancy but cannot definitively rule it out. Ectopic pregnancies can be missed or misinterpreted on ultrasound, especially in early gestational stages.

Option B (serum progesterone level) is incorrect because progesterone levels are influenced by other factors like menstrual cycle, pregnancy within the uterus, and other conditions. It cannot specifically rule out ectopic pregnancy.

Option D (serum alpha-fetoprotein [AFP] level) is incorrect because AFP is produced primarily in the developing fetus and does not specifically help in diagnosing or ruling out ectopic pregnancy. AFP levels can be influenced by various non-pregnancy-related factors.

**Clinical Pearl / High-Yield Fact:**

In clinical practice, a combination of β-hCG and clinical suspicion is essential for diagnosing ectopic pregnancy. If the serum β-hCG level is normal, further diagnostic tests like transvaginal ultrasound and hemodynamic tests should be considered to rule out ectopic pregnancy. However, if the β-hCG level is increased, further evaluation with ultrasound and hemodynamic tests is crucial to confirm the diagnosis and guide management, as ectopic pregnancy can lead to severe bleeding and life-threatening complications if left untreated.