Maximum diameter of pelvic inlet is :
**Core Concept**
The pelvic inlet, also known as the pelvic brim, is the upper boundary of the pelvic cavity. It is a critical area in obstetrics and gynecology, as it determines the size of the pelvic outlet and the potential for vaginal delivery. The pelvic inlet is formed by the union of the iliopectineal lines of the ilium, the promontory of the sacrum, and the arcuate line of the pubis.
**Why the Correct Answer is Right**
The maximum diameter of the pelvic inlet is an important measurement in determining the size of the pelvic cavity. It is typically measured at the level of the promontory of the sacrum, where the iliopectineal lines meet. The correct answer is based on this anatomical landmark. The maximum diameter of the pelvic inlet is approximately 12-13 cm in most adults, which allows for a relatively easy passage of the fetal head during vaginal delivery.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it is a typical diameter of the pelvic outlet, not the inlet.
* **Option B:** This option is incorrect as it is a typical diameter of the pelvic cavity, not the inlet.
* **Option C:** This option is incorrect as it is a typical diameter of the fetal head, not the pelvic inlet.
**Clinical Pearl / High-Yield Fact**
The size of the pelvic inlet is an important factor in determining the potential for vaginal delivery. A pelvic inlet diameter of less than 12 cm may indicate a higher risk of cephalopelvic disproportion, where the fetal head is too large to pass through the pelvic inlet.
**Correct Answer: C. 12-13 cm**