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?

Choice A reason: Serotonin deficiency is a well-established biological risk factor for major depressive disorder. It affects mood regulation and is a target for many antidepressant therapies. Choice B reason: Acute bronchitis is a temporary respiratory condition and is not associated with increased risk for depression. Chronic illnesses may contribute, but acute bronchitis alone is not a recognized risk factor. Choice C reason: Elevated calcium levels may indicate hyperparathyroidism or other metabolic issues, but they are not directly linked to major depressive disorder as a primary risk factor. Choice D reason: Being an only child is not a validated risk factor for depression. While social isolation or lack of support may contribute, birth order or sibling status alone does not determine risk.