Differential Diagnosis

The patient presents with marked swelling of the labia majora, overlying erythema, and a history of self-catheterization as a paraplegic. The laboratory and imaging findings provide clues to the possible diagnoses. Here's the analysis organized into the requested categories:

• Single Most Likely Diagnosis
  • Urinary Tract Infection (UTI) or Periurethral Abscess: The presence of nitrite-positive urinalysis suggests a UTI, which is common in individuals performing self-catheterization. The swelling and erythema of the labia majora could be indicative of a periurethral abscess, a complication of UTI, especially given the patient's history of catheterization.
• Other Likely Diagnoses
  • Vulvar Cellulitis: The erythema and swelling of the labia majora could also be consistent with vulvar cellulitis, an infection of the skin and subcutaneous tissue. This condition can occur in the absence of a specific wound or break in the skin, especially in immunocompromised or diabetic patients, but the patient's paraplegia and catheterization history increase the risk.
  • Vaginal Abscess: Although the wet prep is negative, a vaginal abscess could still be considered, especially if there's a history of recent vaginal procedures or infections. However, the primary complaint and findings are more suggestive of external genitalia involvement.
• Do Not Miss Diagnoses
  • Necrotizing Fasciitis: This is a life-threatening condition that requires immediate recognition and treatment. Although less likely given the current presentation and lack of systemic signs of severe infection (e.g., high fever, severe pain out of proportion to examination), it cannot be entirely ruled out without close monitoring and possibly further diagnostic procedures.
  • Septic Pelvic Thrombophlebitis: This condition, although rare, can occur in the setting of pelvic infections and could explain systemic symptoms if present. The absence of significant systemic symptoms and the lack of specific findings on CT scan make this less likely but not impossible.
• Rare Diagnoses
  • Crohn's Disease or Other Inflammatory Bowel Diseases with Genital Involvement: These conditions can rarely present with genital manifestations, including swelling and abscesses, but are less likely given the patient's presentation and the absence of gastrointestinal symptoms.
  • Hidden Malignancy: Although extremely rare and not directly suggested by the current workup, any unexplained or persistent genital swelling should prompt consideration of malignancy, especially in the absence of a clear infectious cause.

Treatment Approach

Based on the most likely diagnosis of a UTI or periurethral abscess, treatment should include:

• Broad-spectrum antibiotics to cover common urinary pathogens, considering the possibility of resistant organisms due to frequent catheterization.
• Imaging (possibly a dedicated pelvic ultrasound) to confirm the presence of an abscess, which may require drainage.
• Optimization of catheterization technique and consideration of alternative methods to reduce infection risk.
• Close follow-up to monitor for resolution of symptoms and to adjust the antibiotic regimen based on culture results, if obtained.

For other diagnoses, treatment would be tailored to the specific condition, emphasizing the importance of a thorough diagnostic workup to guide therapy.