**Core Concept**
Fluoxetine, a selective serotonin reuptake inhibitor (SSRI), is used to treat various psychiatric disorders. Its mechanism of action involves increasing the concentration of serotonin in the synaptic cleft by inhibiting the reuptake of serotonin by the presynaptic neuron.

**Why the Correct Answer is Right**
The most common side effect of fluoxetine therapy is nausea, which occurs due to the increased levels of serotonin in the gut. This effect is mediated by the 5-HT3 receptors, which are present in the enteric nervous system. The increased serotonin binding to these receptors stimulates the vomiting center in the brain, leading to nausea and vomiting.

**Why Each Wrong Option is Incorrect**
* **Option A:** While fluoxetine can cause weight gain, it is not the most common side effect.
* **Option B:** Fluoxetine is not commonly associated with increased heart rate or blood pressure.
* **Option C:** Although fluoxetine can cause sexual dysfunction, it is not the most common side effect.

**Clinical Pearl / High-Yield Fact**
SSRIs like fluoxetine work by increasing serotonin levels, but they can also cause serotonin syndrome, a potentially life-threatening condition, when combined with other serotonergic medications.

**Correct Answer: B. Nausea**