**Core Concept**
Body dysmorphic disorder (BDD) is a mental health condition characterized by a preoccupation with a perceived flaw in one's appearance, which is not observable or appears slight to others. This condition is often associated with excessive concern about physical appearance, leading to significant distress and impairment in daily life.

**Why the Correct Answer is Right**
The correct diagnosis is body dysmorphic disorder (BDD), which is a type of obsessive-compulsive disorder (OCD). Individuals with BDD experience intrusive thoughts and obsessive preoccupations about a perceived flaw in their appearance, often leading to repetitive behaviors such as excessive mirror-checking, seeking reassurance, or undergoing cosmetic procedures to correct the perceived defect. In this case, the patient's preoccupation with the size of her nose and her distress about it, despite no one else noticing it, are indicative of BDD.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a specific diagnosis that matches the patient's symptoms. While anxiety disorders can present with excessive concern about physical appearance, BDD is a more specific diagnosis that requires a preoccupation with a perceived flaw in one's appearance.
**Option B:** This option is incorrect because social anxiety disorder typically involves fear of social situations or being judged by others, rather than a preoccupation with a perceived flaw in one's appearance.
**Option C:** This option is incorrect because obsessive-compulsive disorder (OCD) typically involves intrusive thoughts and compulsions to perform specific rituals, whereas BDD is a specific type of OCD that focuses on a perceived flaw in one's appearance.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that individuals with BDD often have a distorted body image and may exhibit excessive reassurance-seeking behavior. A thorough mental health evaluation, including a review of the patient's symptoms and behavior, is necessary to diagnose BDD accurately.

**Correct Answer:** C.