**Core Concept**
Ectopic pregnancy occurs when a fertilized ovum implants outside the uterine cavity, most commonly in the fallopian tube. This condition poses significant risks to the mother's life and the developing embryo, necessitating prompt diagnosis and intervention.

**Why the Correct Answer is Right**
The correct answer is **Option D:** Hematometra, or the presence of blood in the uterine cavity, is a pathognomonic feature of ectopic pregnancy. This occurs due to the rupture of the fallopian tube, causing bleeding into the peritoneal cavity and subsequently into the uterine cavity. The bleeding can be detected through ultrasound or endometrial aspiration, confirming the diagnosis of ectopic pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Irregular menstrual bleeding is a nonspecific symptom that can be associated with various conditions, including ectopic pregnancy, but it is not pathognomonic.

**Option B:** Pelvic pain is a common symptom of ectopic pregnancy but can also be present in other conditions such as ovarian torsion, endometriosis, or appendicitis.

**Option C:** Abdominal tenderness is a nonspecific sign that can be present in various abdominal conditions, including ectopic pregnancy, but it is not a definitive diagnostic feature.

**Clinical Pearl / High-Yield Fact**
In cases of suspected ectopic pregnancy, it is essential to perform a beta-hCG level to assess the pregnancy's viability and to follow up with ultrasound to confirm the location of the embryo.

**Correct Answer:** D. Hematometra.