A nurse is reviewing the medical record of a newly admitted client who has major depressive disorder. Which of the following findings should the nurse identify as a risk factor for this condition?

A. The client has a serotonin deficiency. Serotonin plays a crucial role in mood regulation, and low levels of serotonin are strongly associated with major depressive disorder (MDD). This neurotransmitter imbalance contributes to symptoms such as persistent sadness, low energy, and disrupted sleep patterns. Selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed to increase serotonin levels and alleviate depressive symptoms. B. The client has acute bronchitis. While physical illnesses can impact mood, acute bronchitis is a temporary respiratory infection that does not directly cause major depressive disorder. Chronic illnesses may contribute to depression, but acute conditions are not considered significant risk factors. C. The client has an elevated calcium level. Hypercalcemia can cause fatigue, confusion, and depression-like symptoms, but it is a medical condition rather than a primary cause of MDD. Treating the underlying cause of hypercalcemia usually resolves mood-related symptoms. D. The client is an only child. Birth order and family size have no direct biological or psychological link to MDD. While loneliness or social factors can influence mental health, being an only child is not considered a risk factor for major depressive disorder.