**Core Concept**
Premenstrual dysphoric disorder (PMDD) is a severe form of premenstrual syndrome (PMS) characterized by emotional and physical symptoms that occur in the luteal phase of the menstrual cycle, typically resolving with the onset of menses. The pathophysiology involves hormonal fluctuations, particularly the relative progesterone withdrawal and increased estrogen levels.

**Why the Correct Answer is Right**
Fluoxetine, a selective serotonin reuptake inhibitor (SSRI), is a first-line treatment for PMDD. It works by increasing serotonin levels in the brain, which helps regulate mood, emotional responses, and hormonal changes. SSRIs like fluoxetine are effective in reducing symptoms of PMDD, including emotional liability, depression, and anxiety. The mechanism of action is thought to involve the inhibition of serotonin reuptake, increasing the availability of serotonin in the synaptic cleft, which in turn helps alleviate PMDD symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Evening primrose oil, rich in gamma-linolenic acid (GLA), is sometimes used to treat PMS symptoms, but its effectiveness in PMDD is limited and not supported by robust evidence.
**Option B:** Vitamin B6, also known as pyridoxine, has been studied as a potential treatment for PMS symptoms, but its efficacy in PMDD is unclear, and it is not a recommended first-line treatment.
**Option D:** Progesterone supplementation is sometimes used to treat PMS symptoms, but it is not effective in treating PMDD, as the condition is more closely related to the relative progesterone withdrawal and increased estrogen levels.

**Clinical Pearl / High-Yield Fact**
SSRIs like fluoxetine are effective in treating PMDD due to their ability to regulate serotonin levels, which helps alleviate symptoms of emotional liability, depression, and anxiety. It's essential to diagnose PMDD accurately, as the condition can be misdiagnosed as depression or anxiety disorders.

**✓ Correct Answer: C. Fluoxetine**