**Core Concept**
Beta-hCG is a hormone produced by the placenta during pregnancy, and its detection is crucial for diagnosing early pregnancy. A blood test measuring beta-hCG levels is essential in evaluating a patient presenting with amenorrhea and vaginal spotting, as these symptoms can be indicative of pregnancy or other pathologies.

**Why the Correct Answer is Right**
The most useful diagnostic test in this scenario is the measurement of beta-hCG levels in the blood. This test helps determine whether the patient is pregnant and can also aid in the diagnosis of ectopic pregnancy, which is a life-threatening condition. The beta-hCG test is particularly useful in the first trimester of pregnancy, as it can detect even small amounts of the hormone in the blood. A high beta-hCG level can indicate a normal pregnancy, while a low level or no rise in beta-hCG levels over time may suggest an ectopic pregnancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ultrasound is a useful diagnostic tool in evaluating a patient with amenorrhea and vaginal spotting, but it is not the most useful initial test. Ultrasound can confirm a pregnancy, but it may not detect an ectopic pregnancy, especially in the early stages.
* **Option B:** A pelvic examination may provide some clues, but it is not a reliable diagnostic test for early pregnancy. A pelvic exam may not detect an ectopic pregnancy, and it can also be misleading if the patient has a normal pregnancy.
* **Option D:** A pregnancy test (urine hCG) is a quick and simple test, but it is not as sensitive as a blood test measuring beta-hCG levels. A urine test may not detect an ectopic pregnancy or a pregnancy in its early stages.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that beta-hCG levels typically double every 48 hours in a normal pregnancy. If the beta-hCG levels do not rise as expected, it may indicate an ectopic pregnancy or a failed pregnancy.

**Correct Answer: C.** Beta-hCG blood test