**Core Concept**
The pelvic diameter is a critical anatomical measurement in obstetrics, determining the ease of vaginal delivery. The shortest pelvic diameter is a key factor in assessing the likelihood of cephalopelvic disproportion.

**Why the Correct Answer is Right**
The shortest pelvic diameter is the interspinous diameter, which measures the distance between the ischial spines. This diameter is crucial because it represents the narrowest part of the pelvis. The interspinous diameter is approximately 10-11 cm in adult females, making it the shortest pelvic diameter.

**Why Each Wrong Option is Incorrect**
**Option A:** The obstetric conjugate is the diagonal conjugate, which is approximately 11-12 cm. It is not the shortest pelvic diameter.
**Option B:** The subpubic angle is a measurement of the pelvic inlet, but it is not a diameter. It is approximately 90-100 degrees.
**Option C:** The inlet of the pelvis is the widest part, and the diagonal conjugate is the longest diameter, making it incorrect.

**Clinical Pearl / High-Yield Fact**
Remember the order of pelvic diameters from shortest to longest: interspinous, obstetric conjugate, transverse diameter, and diagonal conjugate.

**Correct Answer: C. The interspinous diameter.**