**Core Concept**
The antero-posterior (AP) diameter of the pelvic inlet is a crucial measurement in obstetrics and gynecology. It is the distance from the promontory of the sacrum to the pubic symphysis, representing the maximum width of the pelvic cavity.

**Why the Correct Answer is Right**
The longest AP diameter of the pelvic inlet is the Oblique Diameter (also known as the Oblique Interspinous Diameter). This measurement is taken from the sacral promontory to the upper margin of the pubic symphysis. The Oblique Diameter is the maximum AP diameter because it traverses the widest portion of the pelvic inlet, which is the area where the sacrum and pubic bone meet. This diameter is essential in determining the suitability of the pelvis for vaginal delivery.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is not a recognized AP diameter of the pelvic inlet.
* **Option B:** The True Conjugate is the shortest AP diameter of the pelvic inlet, measured from the lower margin of the pubic symphysis to the sacral promontory.
* **Option C:** The Interspinous Diameter is not the longest AP diameter of the pelvic inlet; it is a smaller diameter measured between the ischial spines.

**Clinical Pearl / High-Yield Fact**
In obstetrics, knowledge of the pelvic inlet diameters is crucial for assessing the suitability of the pelvis for vaginal delivery. A pelvis with a narrow inlet diameter may require a cesarean section.

**Correct Answer: C. Oblique Diameter.