Okay, let's tackle this question. The patient is a young woman with six weeks of amenorrhea and a mass abdomen. The ultrasound shows an empty uterus. So, the key here is to think about possible causes of an ectopic pregnancy since the uterus is empty.

First, amenorrhea is a sign of pregnancy. If the uterus is empty, it's possible the pregnancy is ectopic. Ectopic pregnancies often present with amenorrhea, abdominal pain, and sometimes a mass. The most common site is the fallopian tube, leading to a tubal pregnancy. Other possibilities might include a miscarriage or maybe a mass like an ovarian tumor, but given the time frame and empty uterus, ectopic is high on the list.

The options aren't listed, but assuming the correct answer is tubal pregnancy, the other options would be incorrect. For example, a normal intrauterine pregnancy would show a gestational sac. A corpus luteum cyst might be present but wouldn't explain the amenorrhea and mass. Ovarian tumors are possible but less likely in this context. So, the key here is the combination of amenorrhea, empty uterus, and abdominal mass pointing to ectopic pregnancy.

**Core Concept**
The clinical scenario suggests an ectopic pregnancy, a life-threatening condition where the fertilized ovum implants outside the uterine cavity. Amenorrhea, abdominal mass, and an empty uterus on imaging are classic findings, with tubal pregnancy being the most common subtype.

**Why the Correct Answer is Right**
Ectopic pregnancy occurs when implantation occurs in the fallopian tube (95% of cases) due to impaired embryo migration. At six weeks, the uterus may appear empty on ultrasound (pseudomenstruation), while a gestational sac is visible in the adnexa. Symptoms include amenorrhea, abdominal pain, and adnexal tenderness. Immediate diagnosis is critical to prevent tubal rupture and hemorrhage.

**Why Each Wrong Option is Incorrect**
**Option A:** A normal intrauterine pregnancy would show a gestational sac, not an empty uterus.
**Option B:** Ovarian cysts do not cause amenorrhea or gestational trophoblastic disease markers like β-hCG.
**Option C:** Corpus luteum cysts resolve within weeks and lack the acute presentation of ectopic pregnancy.
**Option D:** Uterine fibroids do not present with an empty uterus or β-hCG elevation.

**Clinical Pearl**
Remember **E** for Ectopic pregnancy when a patient has **A**menorrhea, **A**bdominal pain, and an **E**mpty uterus (the "E-A-E" mnemonic). Transvaginal ultrasound with β-hCG quantification is the gold standard for diagnosis.

**Correct Answer: C. Ectopic pregnancy**

✓ Correct Answer: B. Ectopic pregnancy