**Core Concept**
Mittelschmerz is a clinical phenomenon associated with ovulation, characterized by pelvic pain that occurs around mid-cycle, due to the rupture of the dominant follicle and the release of an ovum. This event triggers an inflammatory response in the peritoneal cavity.

**Why the Correct Answer is Right**
The correct answer, **B. Peritoneal irritation from corpus hemorrhagicum**, is the most accurate description of mittelschmerz. During ovulation, the dominant follicle ruptures, releasing the ovum and leading to the formation of a corpus hemorrhagicum, a blood-filled structure in the ovary. This process irritates the peritoneal lining of the pelvic cavity, causing pain. The peritoneal irritation is a direct consequence of the corpus hemorrhagicum, making option B the correct answer.

**Why Each Wrong Option is Incorrect**
**Option A:** The fern pattern of cervical mucus is a characteristic feature of estrogen-dominant cervical mucus, typically observed during the follicular phase of the menstrual cycle. It is not directly related to mittelschmerz.
**Option C:** The rise of body temperature due to the effect of progesterone is a feature of the luteal phase, not mittelschmerz. Progesterone levels increase after ovulation, causing a rise in basal body temperature.
**Option D:** The LH surge before ovulation is a separate event that triggers ovulation, but it is not the cause of mittelschmerz. The LH surge occurs earlier in the cycle, and mittelschmerz occurs around mid-cycle, after ovulation has taken place.

**Clinical Pearl / High-Yield Fact**
Mittelschmerz is often mistaken for other causes of pelvic pain, such as endometriosis or ovarian torsion. However, mittelschmerz typically resolves on its own within a few hours and is often accompanied by a mild, dull ache on one side of the pelvis.

✓ Correct Answer: B. Peritoneal irritation from corpus hemorrhagicum