AN49.1-5 | Perineum — Gate Quiz

Correct. At least five sets of muscles converge on the perineal body: (1) superficial transverse perinei (bilateral), (2) deep transverse perinei (bilateral), (3) bulbospongiosus, (4) external anal sphincter (anterior fibres), and (5) levator ani (puborectalis, pubococcygeus). This makes it the structural keystone of the pelvic floor.

The perineal body is the convergence point of multiple perineal and pelvic floor muscles: superficial + deep transverse perinei, bulbospongiosus, external anal sphincter, and levator ani. This is why its injury during childbirth has such wide-ranging consequences for pelvic floor integrity.

Correct. Colles' fascia (superficial perineal fascia) is continuous with Scarpa's fascia of the anterior abdominal wall. Its lateral attachment to the fascia lata at the inguinal ligament creates a barrier preventing spread to the thigh. Blood (or urine) deep to Colles' fascia therefore tracks anteriorly and superiorly into the scrotum and anterior abdominal wall, not into the thigh.

Colles' fascia (perineum) and Scarpa's fascia (anterior abdominal wall) are continuous. The critical anatomical fact is that Colles' fascia attaches to the fascia lata (deep fascia of the thigh) at the inguinal ligament — creating a fascial boundary that prevents extravasation into the thigh. This explains the clinical distribution of extravasation.

Correct. Alcock's canal (pudendal canal) is a fascial tunnel formed within the obturator fascia on the lateral wall of the ischiorectal fossa. It runs anteriorly from just below the lesser sciatic foramen to the posterior edge of the perineal membrane. It contains the pudendal nerve and internal pudendal vessels.

Alcock's (pudendal) canal is a splitting of the obturator fascia on the LATERAL wall of the ischiorectal fossa. The pudendal nerve re-enters the perineum through the lesser sciatic foramen and then travels forward in this fascial tunnel to supply the entire perineum.

Correct. Goodsall's rule: posterior external opening → track curves to the posterior midline (6 o'clock) of the dentate line — often as a horseshoe extension through the deep postanal space. Anterior external opening → straight (radial) track to the nearest point on the dentate line. This rule guides surgical planning and predicts the internal opening.

Goodsall's rule: Anterior external opening = straight radial track to dentate line. Posterior external opening = curved track to posterior midline of the dentate line (6 o'clock). Remembering this saves time in finding the internal opening during fistulotomy.

Correct. Bartholin's glands (greater vestibular glands) are in the superficial perineal pouch, adjacent to the posterior end of the vaginal orifice. The pudendal nerve (S2–S4) exits the lesser sciatic foramen and runs forward in Alcock's canal on the lateral wall of the ischiorectal fossa, giving off all perineal branches. A pudendal nerve block at the ischial spine (transvaginal approach) or Alcock's canal anaesthetises the labia, clitoris, perineum, and lower vagina.

Bartholin's glands lie in the superficial perineal pouch. The pudendal nerve is the primary nerve supply to the entire perineum and is blocked by injecting near the ischial spine (transvaginal) or in Alcock's canal. This approach provides wide-field anaesthesia for perineal procedures.

Correct. The classification of perineal tears: 1st degree = skin only; 2nd degree = skin + perineal muscles (not sphincter); 3rd degree = external anal sphincter (3a: <50%, 3b: >50%, 3c: internal sphincter also); 4th degree = external anal sphincter + internal anal sphincter + anal mucosa (full-thickness). 4th-degree tears require immediate repair in theatre.

4th-degree tear = the most severe obstetric perineal injury: involves the external AND internal anal sphincters PLUS the anal mucosa (full-thickness anorectal injury). Unrepaired or poorly repaired 4th-degree tears cause faecal incontinence — a devastating and preventable condition.

Correct. The deep postanal space (behind the anal canal, deep to the anococcygeal ligament, connecting the two ischiorectal fossae) allows a perianal abscess to spread from one side to the other — creating a horseshoe abscess. This requires incision and drainage of both fossae AND the posterior midline — if only one side is drained, the abscess recurs from the contralateral extension.

The deep postanal space is the communication between the two ischiorectal fossae. A horseshoe abscess uses this route to spread from one side to the other. Treatment requires bilateral drainage (counter-incisions) + drainage of the posterior midline component — otherwise recurrence is certain.

Correct. The posterior commissure of the anal canal has a relatively poor arterial blood supply (terminal branches of the inferior rectal arteries meet here but with least overlap). Combined with the maximum shearing stress during passage of faeces, this makes it the most vulnerable site for fissure formation. Chronic internal sphincter spasm further reduces blood flow, perpetuating the fissure.

Posterior midline anal fissure is explained by: (1) relative ischaemia — least arterial supply at 6 o'clock; (2) maximum mechanical stress — the posterior commissure is the most stretched during defaecation; (3) internal sphincter hypertonia — reduces blood flow further. Lateral internal sphincterotomy targets this spasm.

Correct. The deep perineal pouch is the space between the perineal membrane (floor) and the pelvic diaphragm (levator ani, roof). It contains: deep transverse perinei muscles, external urethral sphincter (compressor urethrae + sphincter urethrovaginalis in females), pudendal nerve and its branches, and internal pudendal vessels.

Deep perineal pouch (between perineal membrane below and pelvic diaphragm above) = deep transverse perinei + EUS + pudendal neurovascular bundle. Superficial perineal pouch (between perineal membrane above and Colles' fascia below) = erectile bodies + bulbospongiosus + ischiocavernosus + superficial transverse perinei.

Correct. Goodsall's rule: an external opening POSTERIOR to the transverse anal line → the track curves to open at the POSTERIOR midline (6 o'clock) of the dentate line. (Note: Crohn's disease is an exception — Crohn's fistulae can have multiple tracks and multiple openings; however, the question asks about Goodsall's rule application.)

Goodsall's rule for a posterior external opening: the internal opening is at the posterior midline (6 o'clock) of the dentate line. The track curves (not straight). This is in contrast to an anterior external opening, which has a straight radial track to the nearest point on the dentate line. (Crohn's disease can override this rule with complex multiple tracks.)