**Core Concept**
The diagnosis of Pelvic Inflammatory Disease (PID) requires a combination of clinical findings, laboratory results, and imaging studies. The gold standard investigation in diagnosing PID involves identifying the causative organism and confirming the extent of infection.

**Why the Correct Answer is Right**
The gold standard investigation for diagnosing PID is laparoscopy, which allows for direct visualization of the pelvic organs and the collection of tissue samples for histopathological examination and culture. Laparoscopy is particularly useful in cases where the diagnosis is unclear or when there is a high suspicion of tubo-ovarian abscess or other complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Imaging studies such as ultrasound or CT scans are not the gold standard for diagnosing PID, although they can provide valuable information about the extent of infection and the presence of complications.
**Option B:** Clinical diagnosis alone is not sufficient for diagnosing PID, as it relies on symptoms and signs that can be nonspecific and variable.
**Option C:** Blood cultures are not the gold standard for diagnosing PID, as they may not always detect the causative organism, especially in cases of anaerobic or fastidious organisms.

**Clinical Pearl / High-Yield Fact**
Laparoscopy is the gold standard for diagnosing PID because it allows for direct visualization and sampling of the affected tissues, which can be crucial in guiding treatment and preventing long-term complications.

**Correct Answer:** C. Laparoscopy