**Core Concept**
The patient's symptoms of excessive worry, anxiety, irritability, and worsening sleep are characteristic of a common psychiatric disorder that affects millions worldwide. This condition is often associated with significant distress and impairment in daily functioning.

**Why the Correct Answer is Right**
The patient's presentation is consistent with Generalized Anxiety Disorder (GAD), which is characterized by excessive and persistent worry about everyday things, even when there is no apparent reason to worry. This worry is often accompanied by symptoms such as restlessness, irritability, difficulty concentrating, and sleep disturbances. The pathophysiology of GAD is thought to involve an imbalance of neurotransmitters, particularly serotonin and GABA, which regulate mood and anxiety. The brain regions involved in GAD include the amygdala, prefrontal cortex, and anterior cingulate cortex.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Obsessive-Compulsive Disorder (OCD) is characterized by recurrent, intrusive thoughts (obsessions) and repetitive behaviors (compulsions) that an individual feels the urge to perform. While anxiety is a common feature of OCD, the patient's symptoms of excessive worry and irritability without compulsions do not align with OCD.

**Option B:** This option is incorrect because Major Depressive Disorder (MDD) is characterized by persistent feelings of sadness, hopelessness, and loss of interest in activities. While anxiety can be a feature of MDD, the patient's primary complaint is excessive worry, which is not a hallmark symptom of depression.

**Option C:** This option is incorrect because Post-Traumatic Stress Disorder (PTSD) is characterized by symptoms that occur after a traumatic event, such as flashbacks, nightmares, and avoidance of triggers. The patient's symptoms of excessive worry and irritability without a reported traumatic event do not align with PTSD.

**Option D:** This option is incorrect because Bipolar Disorder is characterized by episodes of mania or hypomania, which are not mentioned in the patient's presentation.

**Clinical Pearl / High-Yield Fact**
A key feature of GAD is the presence of excessive worry that is not limited to a specific situation or event, but rather is diffuse and pervasive. This can be remembered using the mnemonic "GAD: Generalized Anxiety Disorder with worry that's not Gone Away".

**Correct Answer: C. Post-Traumatic Stress Disorder (PTSD)