Correct Answer: B. Fistula in ano
Fistula in ano is an abnormal tract lined with granulation tissue that connects the anal canal (usually at the level of an anal crypt) to the perianal skin. The 3-year chronicity with recurrent discharge and pain is the discriminating feature—this duration rules out acute suppuration and points to a chronic, epithelialized tract. The clinical presentation of persistent purulent drainage alternating with periods of relative quiescence, combined with pain during defecation and sitting, is pathognomonic for fistula in ano. Most fistulas (90%) originate from infection of anal glands in the crypts of Morgagni, leading to abscess formation that eventually ruptures and drains externally, leaving a persistent tract. The tract may be simple (straight) or complex (with multiple branches, horseshoe configuration). On examination, a visible external opening with granulation tissue and history of recurrent drainage confirms the diagnosis. According to Bailey & Love and Indian colorectal surgical practice, fistula in ano is classified by Parks' classification (intersphincteric, transsphincteric, suprasphincteric, extrasphincteric) based on MRI or examination findings. The chronic nature, recurrent discharge, and perilesional inflammation over years are classic for fistula in ano.
Why the other options are wrong
A. Carbuncle — A carbuncle is an acute, suppurative infection involving multiple hair follicles and subcutaneous tissue, presenting with fever, systemic toxicity, and rapid progression over days to weeks. It is NOT a chronic condition and does not produce recurrent discharge over 3 years. Carbuncles resolve either by drainage or healing; they do not form persistent epithelialized tracts. The 3-year duration rules out this acute pyogenic infection. C. Pilonidal sinus — Pilonidal sinus is a chronic condition but occurs in the sacrococcygeal region (natal cleft), NOT around the anus. It presents with discharge and pain but is unrelated to anal crypts or the anal canal. The location and embryological origin (ingrown hair follicles) are entirely different from fistula in ano. Pilonidal sinus does not communicate with the anal canal, which is the key distinguishing feature. D. Boil — A boil (furuncle) is an acute, localized pyogenic infection of a single hair follicle, presenting with pain, swelling, and pus formation over 1–2 weeks. It is self-limiting and resolves by drainage or healing without leaving a persistent tract. A boil cannot explain 3 years of recurrent discharge and would not form a chronic fistulous tract. The chronicity and recurrent nature are incompatible with simple furuncle pathology.
High-Yield Facts
- Fistula in ano originates from infection of anal glands in crypts of Morgagni in 90% of cases, leading to abscess rupture and tract formation.
- 3-year chronicity with recurrent discharge and pain is pathognomonic for fistula in ano; acute suppurations (boil, carbuncle) resolve within weeks.
- Parks' classification divides fistulas into intersphincteric (45%), transsphincteric (30%), suprasphincteric (20%), and extrasphincteric (5%) based on tract anatomy.
- MRI pelvis is the gold standard for mapping complex fistulas and identifying secondary tracts before surgical planning in Indian practice.
- Surgical management (fistulotomy for simple fistulas, seton placement for complex fistulas) is the definitive treatment; antibiotics alone cannot cure a fistula.
Mnemonics
FISTULA IN ANO vs ACUTE SUPPURATION CHRONIC = Fistula (3+ years, recurrent discharge, epithelialized tract). ACUTE = Boil/Carbuncle (days–weeks, fever, systemic toxicity, resolves without tract). Use this to rule out acute infections when chronicity is mentioned. LOCATION RULE Fistula in ano = around anus (anal canal origin). Pilonidal sinus = sacrococcygeal region (natal cleft). Different locations, different pathology—do not confuse.
NBE Trap
NBE pairs "recurrent discharge" with acute infections (boil, carbuncle) to trap students who focus on the symptom rather than the 3-year chronicity—the key discriminator that shifts diagnosis from acute suppuration to chronic fistula.
Clinical Pearl
In Indian outpatient colorectal clinics, fistula in ano is one of the most common chronic perianal conditions. Patients often present after years of self-treatment with antibiotics and topical agents, which fail because antibiotics cannot sterilize a non-epithelialized tract—only surgical drainage and fistulotomy/seton placement offer cure. Always ask about the duration and pattern of discharge to distinguish chronic fistula from recurrent acute abscess.
_Reference: Bailey & Love Ch. 72 (Anal Fistula); Harrison Ch. 287 (Gastrointestinal Disorders)_