You suspect reactivity of mood in a patient. Before revealing the answer, decide how you would assess for this.

Observe or ask about the patient's mood across a period that includes both positive and negative events or news, noting whether mood brightens and drops in step with what is actually happening. Assess the degree of responsiveness, since genuinely positive events or company should produce a noticeable, sometimes rapid, brightening, while minor setbacks produce a disproportionate drop. Distinguish this from a mood that shifts abruptly without a proportionate real trigger, which would instead point toward lability of affect. Consider the overall pattern across the assessment: reactivity of mood should track circumstances, rather than appearing unstable or unpredictable regardless of what is happening.