## **Core Concept**
The patient's presentation of amenorrhea and a lump in the right iliac fossa suggests a possible ectopic pregnancy, which is a pregnancy complication where the embryo implants outside the uterus, commonly in the fallopian tube. This condition requires prompt diagnosis and treatment. The investigation of choice should ideally be quick, non-invasive, and sensitive enough to confirm or rule out pregnancy and locate the embryo.

## **Why the Correct Answer is Right**
Ultrasound, specifically transvaginal ultrasound, is the investigation of choice in this scenario. It is highly sensitive and specific for detecting intrauterine pregnancies and can also identify ectopic pregnancies. It allows for the assessment of the adnexa and the detection of any abnormalities such as an ectopic pregnancy, ovarian cysts, or other masses. Given that the patient is presenting with amenorrhea and a palpable mass in the right iliac fossa, a transvaginal ultrasound can quickly provide critical information that guides immediate management.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a urine pregnancy test can confirm pregnancy, it does not provide information on the location of the pregnancy. A positive test would necessitate further investigation but does not directly address the location of the embryo.
- **Option B:** A complete blood count (CBC) may provide supportive information, such as evidence of anemia or infection, but it does not directly diagnose the cause of amenorrhea or the nature of the lump.
- **Option C:** Although imaging of the abdomen and pelvis could provide some information, a plain X-ray would not be as sensitive or specific as ultrasound for detecting an early pregnancy or an ectopic pregnancy.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in cases of suspected ectopic pregnancy, a transvaginal ultrasound should be performed with the patient in a stable condition. A serum beta-hCG level is often checked alongside; an intrauterine pregnancy is typically visible on ultrasound when the beta-hCG level reaches 1,000-2,000 IU/L. However, an ectopic pregnancy can be present even with lower levels, emphasizing the need for careful ultrasound evaluation.

## **Correct Answer:** . Ultrasound