Differential Diagnosis for Perianal Itching

• Single most likely diagnosis
  • Pruritus ani: This condition is characterized by intense itching around the anus, which can be exacerbated by moisture, as described by the patient. The history of scratching leading to excoriation and softened skin, along with the absence of a rash, supports this diagnosis. The patient's previous episodes of vaginal candidiasis and the onset of symptoms after her menstrual period could suggest a possible fungal component, but the primary complaint of perianal itching without a described rash points more directly to pruritus ani.
• Other Likely diagnoses
  • Perianal candidiasis: Given the patient's history of recurrent vaginal candidiasis, it's plausible that she could experience perianal candidiasis, especially considering the symptoms worsen with moisture. However, the absence of a described rash and the specific mention of itching without other typical fungal infection symptoms (like redness or discharge) makes this less likely than pruritus ani.
  • Hemorrhoidal irritation: The patient has a history of hemorrhoids and an external hemorrhoid was noted on examination. Although the hemorrhoid was described as non-tender and not inflamed, irritation from the hemorrhoid could potentially contribute to the itching, especially if there's any associated leakage or moisture.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
  • Anal or rectal cancer: Although rare and less likely given the patient's age and presentation, any persistent or severe anal symptoms warrant consideration of malignancy to avoid a delayed diagnosis.
  • Inflammatory bowel disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause perianal symptoms including itching, especially if there are fistulas or abscesses present. The patient's symptoms and history do not strongly suggest IBD, but it's a diagnosis that could have significant implications if missed.
• Rare diagnoses
  • Threadworms (Enterobius vermicularis): This parasitic infection can cause intense perianal itching, typically worse at night. It's more common in children but can occur in adults, especially in cases of poor hygiene or in institutional settings.
  • Psoriasis or other dermatological conditions: Although the patient denies any rash, conditions like psoriasis can cause perianal itching and skin changes. The absence of a described rash and the specific characteristics of the itching make this less likely, but it remains a consideration in the differential diagnosis.