**Core Concept**
Mittelschmerz pain refers to a type of pelvic pain experienced by some women during the ovulatory phase of their menstrual cycle. This pain is often unilateral and can be accompanied by other symptoms such as vomiting and nausea. The exact cause of mittelschmerz pain is not fully understood, but it is thought to be related to the release of an ovum from the ovary.

**Why the Correct Answer is Right**
Mittelschmerz pain typically occurs around the time of ovulation, which is approximately halfway through the menstrual cycle. This pain is believed to be caused by the rupture of the follicle that releases the ovum, leading to inflammation and irritation of the surrounding tissues. The ovary is the site of ovulation, and the pain is often felt on one side of the pelvis, corresponding to the ovary that released the ovum. The release of prostaglandins and other inflammatory mediators also contributes to the pain experienced during mittelschmerz.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because mittelschmerz pain does not occur during the onset of the menstrual cycle, but rather during ovulation, which occurs around mid-cycle.
**Option C:** This option is incorrect because mittelschmerz pain does not occur at the end of the menstrual cycle, but rather during ovulation, which occurs earlier in the cycle.
**Option D:** This option is incorrect because mittelschmerz pain is not related to menstrual bleeding, but rather to ovulation, which occurs before the onset of bleeding.

**Clinical Pearl / High-Yield Fact**
Mittelschmerz pain is often mistaken for appendicitis or other gastrointestinal disorders, so a thorough gynecological history and physical examination are essential in diagnosing this condition. A history of regular menstrual cycles and a clear description of the pain can help distinguish mittelschmerz from other conditions.

**✓ Correct Answer: B. Pain during the usual time of ovulation in menstrual cycle**