**Core Concept**
The true pelvis is the bony pelvis, excluding the pelvic diaphragm, and is divided into two parts: the greater pelvis and the lesser pelvis. The dimensions of the true pelvis are crucial for childbirth and pelvic surgery. The smallest diameter of the true pelvis is a key anatomical landmark.

**Why the Correct Answer is Right**
The true pelvis has several diameters, including the anteroposterior (AP) diameter, the transverse diameter, and the oblique diameter. Among these, the smallest diameter is the interspinous diameter, which is the distance between the ischial spines. This diameter is significant because it is the narrowest point in the true pelvis and is used as a reference point for pelvic measurements. The interspinous diameter is approximately 10 cm in adults.

**Why Each Wrong Option is Incorrect**
**Option A:** The interspinous diameter is not the same as the interspinous distance, which is the distance between the ischial spines and the sacrum.
**Option B:** The subpubic arch diameter is larger than the interspinous diameter and is not the smallest diameter of the true pelvis.
**Option C:** The pelvic inlet diameter is the largest diameter of the true pelvis and is not the smallest.

**Clinical Pearl / High-Yield Fact**
The ischial spines are a critical landmark in the true pelvis, and their location is used to estimate the size of the pelvic outlet. The interspinous diameter is an essential measurement in obstetrics and gynecology, particularly during childbirth.

**Correct Answer: D. Interspinous diameter.**