**Core Concept**
Depression can be a side effect of various medications due to their impact on neurotransmitter systems in the brain. Certain drugs can either decrease or increase the levels of neurotransmitters like serotonin, dopamine, and norepinephrine, leading to depressive symptoms.

**Why the Correct Answer is Right**
Flupenthixol is an antipsychotic medication belonging to the class of thioxanthenes. Unlike many other antipsychotics, flupenthixol has a unique property of also acting as a dopamine agonist. It has a higher affinity for D2 receptors, which are involved in the regulation of motor function and mood. By stimulating these receptors, flupenthixol can actually have a mood-stabilizing effect and is sometimes used in the treatment of depression, particularly in cases of treatment-resistant depression.

**Why Each Wrong Option is Incorrect**
**Option A:** Propanolol is a beta-blocker used to treat hypertension, angina, and certain arrhythmias. However, it can cause depression as a side effect due to its ability to block the reuptake of norepinephrine, leading to a decrease in the levels of this neurotransmitter in the synaptic cleft.

**Option B:** Oral contraceptives have been associated with mood changes, including depression, in some women. This is thought to be due to changes in the levels of sex hormones, particularly estrogen and progesterone, which can affect the brain's neurotransmitter systems.

**Option C:** Reserpine is an antipsychotic and antihypertensive medication that works by depleting the stores of catecholamines, including dopamine, norepinephrine, and serotonin, in the brain. This can lead to a range of side effects, including depression, due to the decreased levels of these neurotransmitters.

**Clinical Pearl / High-Yield Fact**
It's essential to consider the potential for medication-induced depression when prescribing new medications, especially in patients with a history of depression or those taking multiple medications. Regular monitoring of mood and mental status can help identify any potential issues early on.

**✓ Correct Answer: D. Flupenthixol**