## **Core Concept**
Exercise is increasingly recognized as an adjunctive treatment for depression, with a proposed mechanism involving the enhancement of neurotransmitter systems and neuroplasticity. The therapeutic effects of exercise on depression are believed to be mediated through various physiological and psychological pathways.

## **Why the Correct Answer is Right**
The correct answer, **C. Increasing brain-derived neurotrophic factor (BDNF)**, is supported by evidence suggesting that exercise promotes neurogenesis and neuroplasticity by increasing the production of BDNF. BDNF plays a crucial role in the growth, differentiation, and survival of neurons, and its levels are often decreased in individuals with depression. By enhancing BDNF, exercise may help alleviate depressive symptoms.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While exercise does influence the hypothalamic-pituitary-adrenal (HPA) axis, which is dysregulated in depression, stating it "probably acts by" directly modifying the HPA axis oversimplifies the complex mechanisms involved.
- **Option B:** Exercise does have anti-inflammatory effects, and chronic inflammation is linked to depression. However, saying it acts "probably by" reducing inflammation does not directly address the primary mechanisms through which exercise exerts its antidepressant effects.
- **Option D:** Though exercise can improve sleep quality, and sleep disturbances are common in depression, suggesting it "probably acts by" improving sleep does not capture the direct neurobiological mechanisms through which exercise impacts depressive symptoms.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that exercise has been shown to have a positive effect on depression, likely through multiple mechanisms including the increase of BDNF, reduction of inflammation, and improvement of sleep quality. For patients with depression, starting with low-intensity exercise and gradually increasing the intensity can be an effective adjunctive treatment strategy.

## **Correct Answer:** C. Increasing brain-derived neurotrophic factor (BDNF).