Goodsall's rule is for:

Ans. b. Fistula in ano. (Ref LB 26th/pg. 1260)In fistula in ano, Goodsall's rule used to indicate likley position of internal opening according to the position of external openings.FISTULA-IN-ANO# Intersphincteric fistulae (most common type; 45%) do not cross the external sphincter (bar, for the purist, the most medial subcutaneous fibres running below the distal border of the internal sphincter); most commonly they run directly from the internal to the external openings across the distal internal sphincter, but may extend proximally in the intersphincteric plane to end blindly with or without an abscess, or enter the rectum at a second internal opening.# Goodsall's rule used to indicate the likely position of the internal opening according to the position of the external opening(s), is helpful but not infallible.# Endoanal ultrasound, especially with hydrogen peroxide, can also be used to delineate fistulae, although definition of sepsis outside or above the sphincters is limited by the probe's focal range and scarring makes interpretation difficult. Nonetheless, ultrasound, which is more accurate than clinical examination, is useful to determine whether a fistula is relatively straightforward or not.# MRI is acknowledged to be the 'gold standard' for fistula imaging but it is limited by availability and cost and is usually reserved for difficult recurrent cases. The great advantage of MRI is its ability to demonstrate secondary extensions, which may be missed at surgery and which are the cause of persistence . Fistulography and CT both have limitations but are useful techniques if an extrasphincteric fistula is suspected.Anorectal fistulae# Are common and may be simple or complex# Are classified according to their relationship to the anal sphincters# May be associated wlith underlying disease such as tuberculosis or Crohn's disease# Laying open is the surest method of eradication, but sphincter division may result in incontinence