**Core Concept**
The underlying principle being tested is the distinction between various mood disorders, specifically **major depressive disorder** and **adjustment disorder**, based on symptoms and precipitating events. The patient's symptoms of sadness, palpitations, loss of appetite, and insomnia are indicative of a mood disorder.

**Why the Correct Answer is Right**
Given the symptoms and the absence of hopelessness, suicidal thoughts, or a past history of precipitating events, and considering the patient is functioning normally in her job and social life, the diagnosis is likely **adjustment disorder with depressed mood**. This condition is characterized by the development of emotional or behavioral symptoms in response to an identifiable stressor, which the patient seems to be lacking, but the key here is the absence of full criteria for major depression and the presence of a stressor is not always required for adjustment disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** Major depressive disorder typically includes symptoms such as hopelessness, suicidal thoughts, and significant impairment in social or occupational functioning, which this patient does not exhibit.
**Option B:** Bipolar disorder involves manic or hypomanic episodes, which are not mentioned in the scenario.
**Option C:** Dysthymia is a chronic, low-grade depressive condition that lasts for years, which doesn't fit the acute presentation described.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that adjustment disorders can present with depressive symptoms but without the full criteria for major depression, and they do not necessarily require an identifiable stressor. The ability to function in daily life despite symptoms is a crucial distinguishing factor.

**Correct Answer:** D. Adjustment Disorder.