**Core Concept**
The true pelvis is the part of the pelvis that is bounded by the pelvic brim (pelvic inlet) and the pelvic floor (pelvic outlet). The dimensions of the true pelvis are essential for understanding obstetric delivery and pelvic floor disorders. The interspinous diameter is a critical dimension that affects the passage of the fetal head during labor.

**Why the Correct Answer is Right**
The interspinous diameter is the smallest diameter of the true pelvis, measuring approximately 10-12 cm. This dimension is crucial because it represents the distance between the ischial spines, which is the narrowest part of the pelvic outlet. During labor, the fetal head must pass through the interspinous diameter, and any obstruction or narrowing of this diameter can lead to difficult or obstructed labor. The interspinous diameter is smaller than other dimensions such as the true conjugate (12-14 cm) and the diagonal conjugate (12-14 cm).

**Why Each Wrong Option is Incorrect**
**Option B:** Diagonal conjugate is not the smallest diameter of the true pelvis; it measures 12-14 cm.
**Option C:** True conjugate is also not the smallest diameter of the true pelvis; it measures 12-14 cm.
**Option D:** Intertuberous diameter is not a standard measurement of the true pelvis; it is not a recognized diameter in obstetric anatomy.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the interspinous diameter is the narrowest part of the true pelvis, and any abnormalities or narrowing of this diameter can lead to difficult labor or obstructed labor. Obstetricians should always assess this dimension during prenatal care to identify potential risks and develop a delivery plan.

**✓ Correct Answer: A. Interspinous diameter**