**Core Concept:** Abortion is a spontaneous ending of pregnancy before the fetus can survive independently. Abnormalities in the pregnancy can lead to spontaneous abortion. Ectopic pregnancy is a type of abnormal pregnancy where the fertilized egg implants in a location other than the uterine cavity, typically in the fallopian tube.

**Why the Correct Answer is Right:** The correct answer (C) is based on the clinical findings mentioned in the question. The patient reports with amenorrhea (absence of menstrual period), abdominal pain, and bleeding per vaginum (vaginal bleeding). On speculum examination, the bleeding is found to come from os (opening of the uterus). The uterus size is consistent with 10 weeks gestation and the uterus is soft and the os admits one finger, suggesting a presumably viable pregnancy. The most likely diagnosis is ectopic pregnancy (C) because the pregnancy is outside the uterus, and the patient presents with typical symptoms and signs.

**Why Each Wrong Option is Incorrect:**

A. Amenorrhea and vaginal bleeding are present, but the uterus size is not consistent with gestational age (25 years old females usually have a uterus about the size of a walnut in the first trimester).

B. The patient presents with abdominal pain, which is a possible symptom in an ectopic pregnancy, but the diagnosis is not solely based on abdominal pain alone.

D. The patient presents with bleeding per vaginum, which is a possible symptom in ectopic pregnancy, but the diagnosis is not solely based on this symptom alone.

**Clinical Pearl:** In cases of suspected ectopic pregnancy, the patient's history, physical examination, and imaging studies (ultrasound and beta-human chorionic gonadotropin (β-hCG) test) should be performed to confirm the diagnosis.

**Correct Answer:** C. Ectopic pregnancy