**Core Concept**
The patient's symptoms, characterized by intense depressed mood and an inability to enjoy previously pleasurable activities, are indicative of a specific type of depression that is marked by a persistent and pervasive feeling of sadness.

**Why the Correct Answer is Right**
This symptom is known as anhedonia, which is a core feature of Major Depressive Disorder (MDD). Anhedonia is defined as the inability to experience pleasure in activities that were previously enjoyable. In this case, the patient's inability to enjoy previously pleasurable activities suggests that she has lost interest in things that used to bring her joy, which is a hallmark symptom of MDD. This symptom is thought to be related to the dysfunction of the brain's reward system, which is mediated by neurotransmitters such as dopamine and serotonin.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because it refers to a different symptom of depression, specifically the loss of energy or fatigue, which is also a common feature of MDD but not the specific symptom described in the question.

**Option B:**
This option is incorrect because it refers to a type of anxiety disorder, specifically social anxiety disorder, which is characterized by excessive fear or anxiety in social situations, not a depressed mood or anhedonia.

**Option C:**
This option is incorrect because it refers to a type of personality disorder, specifically borderline personality disorder, which is characterized by instability in relationships, emotions, and behavior, not a depressed mood or anhedonia.

**Clinical Pearl / High-Yield Fact**
It's worth noting that anhedonia is a key distinguishing feature between major depressive disorder and other types of depression, such as dysthymia, which is characterized by a persistent low mood but not necessarily anhedonia.

**Correct Answer: C.**