**Core Concept:** The ilio-pectineal line is a bony landmark that connects the iliac crest to the pectineal line. It is an important anatomical reference point in assessing various measurements and differences in the lower abdominal and pelvic region.

**Why the Correct Answer is Right:** The ilio-pectineal line is primarily used to determine differences in the following:

1. **Option C: The distance between the iliac crest and the pubic symphysis.** This measurement is significant in assessing pelvic trauma, positioning during surgery, and evaluating lower abdominal pain.
2. **Option D: The distance between the iliac crest and the pubic tubercle.** This measurement is used in assessing hip joint function, positioning during surgery, and diagnosing pelvic trauma.

**Why Each Wrong Option is Incorrect:**

1. **Option A:** The inguinal ligament, though also related to the iliac crest, is used to determine the distance between the inguinal canal and the iliac crest. This measurement is significant in assessing inguinal hernia, positioning during surgery, and identifying inguinal canal abnormalities.
2. **Option B:** The pubic symphysis is not directly related to the ilio-pectineal line. Instead, the pubic symphysis is used to determine the distance between the pubic symphysis and the iliac crest, which is relevant to assessing pubic symphysis disorders, positioning during surgery, and diagnosing pelvic trauma.

**Clinical Pearl:** Understanding the ilio-pectineal line and its associated measurements is crucial for various clinical applications, including positioning during surgeries, assessing pelvic trauma, and diagnosing lower abdominal and pelvic disorders. It highlights the importance of recognizing and utilizing key anatomical landmarks for accurate clinical evaluation and management of lower abdominal and pelvic conditions.