Most Likely Diagnosis: Pilonidal Cyst
The most likely diagnosis is A. Pilonidal cyst, given the classic presentation of a painful, fluctuant mass specifically located in the gluteal fold—the pathognomonic location for pilonidal disease.
Clinical Reasoning
Why Pilonidal Cyst is Most Likely
Location is diagnostic: The gluteal fold (intergluteal cleft/natal cleft) is the characteristic anatomic site for pilonidal disease, distinguishing it from other differential diagnoses 1.
Classic presentation triad: The combination of pain, fluctuance (indicating abscess formation), and gluteal fold location represents the typical acute presentation of an infected pilonidal cyst 1.
Age and demographics: Pilonidal disease typically presents in the third and fourth decades of life, with peak incidence in young adults 2.
Why Other Options Are Less Likely
B. Intersphincteric abscess:
- These occur between the internal and external anal sphincters, presenting with perianal (not gluteal fold) pain 3.
- Patients typically have pain with defecation and may have fever, which is not specified in this presentation 3.
- Location is wrong—intersphincteric abscesses are perianal, not in the gluteal fold 3.
C. Dermoid cyst:
- The American College of Radiology specifically notes that dermoid cysts are painless and non-fluctuant, directly contradicting this patient's presentation 1.
- Dermoid cysts are typically present since birth or early childhood, not presenting acutely in adults 1.
- The painful, fluctuant nature of this mass excludes dermoid cyst as a diagnosis 1.
D. Hidradenitis suppurativa:
- While hidradenitis suppurativa can affect the gluteal and perianal regions, it presents with recurrent inflammatory nodules, abscesses, sinus tracts, and scarring in intertriginous areas 4, 5.
- The disease is chronic and characterized by multiple lesions over time, not a single acute painful mass 4.
- Hidradenitis suppurativa affects apocrine gland-rich areas (axillary, inguinal, perianal) but the gluteal fold is not the primary or isolated site 5.
- The question describes a single painful, fluctuant mass without mention of chronicity, recurrence, or sinus tract formation that would suggest hidradenitis suppurativa 4.
Key Clinical Pearls
Anatomic location is the most important diagnostic clue: A painful, fluctuant mass in the gluteal fold is pilonidal disease until proven otherwise 1.
Fluctuance indicates abscess formation: This represents an acute infected pilonidal cyst requiring incision and drainage as initial management 2, 3.
Definitive treatment requires wide excision: While incision and drainage provides acute relief, definitive management involves wide surgical excision to prevent recurrence 2, 3.