**Core Concept**
The patient's presentation of sudden onset severe lower abdominal pain, severe tenderness in the right lower quadrant, and focal enlargement of the proximal right fallopian tube on ultrasound suggests an ectopic pregnancy. This condition occurs when a fertilized egg implants outside the uterus, often in a fallopian tube.

**Why the Correct Answer is Right**
Ectopic pregnancy is often diagnosed through ultrasound and clinical findings, but laboratory findings can also provide crucial information. Beta-human chorionic gonadotropin (β-hCG) is a hormone produced by the placenta during pregnancy. In ectopic pregnancy, β-hCG levels are usually elevated, but the levels may not increase as expected due to the abnormal implantation site. However, the levels may still be high, making it a key marker for ectopic pregnancy diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Progesterone levels may be elevated in early pregnancy, but they are not a specific indicator of ectopic pregnancy.
**Option B:** Urea and electrolyte levels may be abnormal in cases of ectopic pregnancy due to dehydration or hemodynamic instability, but they are not a direct indicator.
**Option C:** Fibrin degradation products (FDPs) may be elevated in cases of ectopic pregnancy due to the release of thromboplastin from the ectopic gestation, but this is not as specific as β-hCG levels.

**Clinical Pearl / High-Yield Fact**
In cases of suspected ectopic pregnancy, the patient's β-hCG levels should be monitored closely, and the levels should be compared to the expected values based on the patient's gestational age. If the levels are not increasing as expected, ectopic pregnancy should be suspected.

**Correct Answer:** C. Fibrin degradation products (FDPs) may be elevated due to the release of thromboplastin from the ectopic gestation.