**Core Concept:** Post-coital bleeding in a 35-year-old female indicates potential endometrial pathology and requires further investigation to determine the cause.

**Why the Correct Answer is Right:** In this scenario, the correct management option is to perform a transvaginal ultrasound (TVS) to assess the endometrial thickness, morphology, and vascularity. TVS allows for precise evaluation of the endometrial lining and can differentiate between various benign and malignant conditions. For instance, if the endometrial thickness is >10mm, it suggests endometrial cancer. If the endometrial lining appears heterogenous or has a polypoidal appearance, it may indicate endometrial hyperplasia. TVS can also assess the uterine cavity for any abnormality, further narrowing down the differential diagnosis.

**Why Each Wrong Option is Incorrect:**

A. Hysterosalpingogram (HSG) is an investigation involving X-ray and contrast medium to assess the uterus and fallopian tubes. It is less specific and sensitive than TVS for evaluating endometrial pathology.

B. Transabdominal ultrasound (TAS) is a less specific and sensitive method for evaluating endometrial pathology compared to TVS. TAS only evaluates the uterus and does not provide detailed information about the endometrial lining.

C. Endometrial biopsy is a diagnostic procedure involving the removal of a small tissue sample from the endometrium. While it can confirm certain diagnoses, it is not the first-line investigation for evaluating post-coital bleeding in this specific context, as it is invasive and requires a more detailed investigation like TVS or hysteroscopy to guide management.

D. Hysteroscopy is an invasive procedure involving the insertion of a camera through the cervix to visualize the endometrial cavity. While hysteroscopy can provide detailed information about the endometrial cavity, it is an invasive procedure that is not the first-line investigation for evaluating post-coital bleeding. TVS is a safer, non-invasive option that provides more comprehensive evaluation of the endometrial lining.

**Clinical Pearl:** Transvaginal ultrasound (TVS) is the preferred first-line investigation for evaluating post-coital bleeding in a 35-year-old female patient due to its non-invasiveness, detailed evaluation, and ability to guide further management.