**Core Concept**
The underlying principle in this question is the management of depression in pregnancy, particularly in the first trimester. The treatment of choice involves balancing the need for effective depression management with the potential risks of medication to the fetus.

**Why the Correct Answer is Right**
The treatment of choice for severe depression in pregnancy, especially in the first trimester, is **Selective Serotonin Reuptake Inhibitors (SSRIs)**. SSRIs are preferred over other antidepressants due to their relatively safer profile for the fetus. They work by increasing the levels of serotonin in the brain, which helps to alleviate symptoms of depression. SSRIs also have a lower risk of causing birth defects compared to other antidepressants.

**Why Each Wrong Option is Incorrect**
* **Option A:** Tricyclic Antidepressants (TCAs) are not the first-line treatment for depression in pregnancy due to their potential for causing birth defects and other side effects.
* **Option B:** Monoamine Oxidase Inhibitors (MAOIs) are not recommended in pregnancy due to the risk of hypertensive crises and other complications.
* **Option C:** Electroconvulsive Therapy (ECT) may be considered in severe cases of depression, but it is not the first-line treatment and carries risks such as memory loss and birth complications.

**Clinical Pearl / High-Yield Fact**
SSRIs are generally considered safe for use in pregnancy, but it's essential to weigh the benefits and risks with the patient and monitor the fetus closely. The risk of birth defects with SSRIs is estimated to be around 1-2%, which is similar to the risk with untreated depression.

**Correct Answer: A. Fluoxetine (Prozac) is an SSRI commonly used in pregnancy.**