**Core Concept**
Pelvic inflammatory disease (PID) is a significant risk factor for infertility due to the potential damage to the fallopian tubes. However, a normal vaginal and cervical examination does not entirely rule out the possibility of tubal damage. The next line of investigation should focus on assessing the patency and function of the fallopian tubes.

**Why the Correct Answer is Right**
In a patient with a history of PID and normal physical examination, the next step would be to perform a hysterosalpingogram (HSG). This radiological investigation involves injecting a contrast agent into the uterus through the cervix, which allows for the assessment of the uterine cavity and fallopian tube patency. The HSG can help identify any blockages or scarring in the fallopian tubes, which may be contributing to the patient's infertility.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound would not provide sufficient information about the patency of the fallopian tubes.
**Option B:** Laparoscopy is a more invasive procedure and would not be the next line of investigation in this case.
**Option C:** Endometrial biopsy would provide information about the endometrial lining but would not assess the fallopian tube patency.

**Clinical Pearl / High-Yield Fact**
In patients with a history of PID, it is essential to investigate the patency of the fallopian tubes to rule out tubal damage as a cause of infertility.

**Correct Answer: D. Hysterosalpingogram.**