**Core Concept**
In the reproductive age group, abnormal uterine bleeding (AUB) is a common symptom that warrants prompt investigation to rule out pregnancy, identify potential causes, and guide further management. The initial investigation should be non-invasive, cost-effective, and capable of detecting pregnancy.

**Why the Correct Answer is Right**
Urine Beta HCG (human chorionic gonadotropin) testing is the first-line investigation for AUB in the reproductive age group. This is because pregnancy is a common cause of AUB, and detecting pregnancy is crucial for management. A urine Beta HCG test is a rapid and non-invasive method to rule out pregnancy, allowing for further evaluation and treatment planning. The sensitivity of urine Beta HCG tests is sufficient to detect pregnancy as early as 6-8 days post-conception.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasonography transabdominal is not the first-line investigation for AUB in the reproductive age group. While ultrasound can be useful in evaluating the uterus and ovaries, it is not as sensitive as urine Beta HCG testing for detecting pregnancy.

**Option B:** A Pap smear is not indicated as an initial investigation for AUB in the reproductive age group. Pap smears are used to screen for cervical cancer and are not relevant to the evaluation of AUB.

**Option D:** Ultrasonography transvaginal can be useful in evaluating the uterus and ovaries, but it is not the first-line investigation for AUB in the reproductive age group. Transvaginal ultrasound is more invasive than urine Beta HCG testing and is not as sensitive for detecting pregnancy.

**Clinical Pearl / High-Yield Fact**
In the reproductive age group, AUB is often caused by ovulatory dysfunction, anovulatory cycles, or pregnancy. Detecting pregnancy is crucial for management, and urine Beta HCG testing is a rapid and non-invasive method to do so.

**Correct Answer Line**
✓ Correct Answer: C. Urine Beta HCG