Final answer:
The 29-year-old woman who began hearing voices after a traumatic event is most likely experiencing acute stress disorder. This condition arises from the recent trauma and accounts for her symptoms, which are consistent with an acute stress response rather than a chronic mental illness such as schizophrenia or mood disorders like dysthymia or bipolar II.
Step-by-step explanation:
The most likely diagnosis for a 29-year-old woman who has started hearing voices since witnessing her child hit by a car 3 weeks ago, and who has also become irritable, fearful, and is having trouble sleeping, is acute stress disorder (B). This diagnosis fits the timeframe of symptoms appearing within four weeks of the trauma and lasting from three days up to one month. Schizophrenia typically presents earlier in life and is a long-term chronic condition, whereas acute stress disorder is a response to a specific stressful event. Dysthymic disorder is a chronic form of depression with lasting symptoms but typically does not include psychotic features like hearing voices. Bipolar II disorder involves cycles of depression and hypomania, not psychosis triggered by trauma. Lastly, adjustment disorder with depressed mood involves emotional and behavioral symptoms in response to a stressful event, but it typically does not include severe symptoms like hallucinations.