🤔 MEDIUM

Test your understanding of the basic pharmacology of the large intestine and some clinical reasoning.

Reviewed by Lena Berger and Jonathan Loomes-Vrdoljak

Pharmacology of the large intestine

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Question 1
A patient on longstanding loperamide presents with new onset constipation.  Which of the following best explains this?
A
Myenteric plexus antagonism
B
Submucosal plexus antagonism
C
Mu-receptor antagonism
D
Kappa-receptor antagonism
Question 1 Explanation: 
Loperamide is a mu receptor agonist - it stimulates opioid mu receptors in the myenteric plexus which slows their activity. Other opioids, like codeine and morphine, also bind to the same receptors, which is why these forms of pain relief can cause constipation.
Question 2
A patient with congenital volvulus has silent bowel sounds in the RLQ. But loud sounds heard in the LUQ. Where is the most likely site of the obstruction?
A
Terminal ileum
B
Ligament of Trietz
C
Ileocaecal valve
D
Midjejunum
Question 3
Which of the following general examination signs indicates late stage constipation?
A
Flat abdomen
B
Localised tenderness
C
Rectal bleeding
D
Enhanced bowel sounds
Question 4
What class of drug is docusate?
A
Stimulant laxative
B
Osmotic laxative
C
Bulk forming laxative
D
Faecal softener
Question 4 Explanation: 
Docusate is a faecal softener. It makes it easier for the stool to hold water and therefore easier to pass. It arguably also has an osmotic laxative effect but it's major mechanism of action it to make the stool easier to move. See DrugBank.
Question 5
Senna works by what mechanism?
A
Increasing the volume of non-absorbable material
B
Increasing stool water content
C
Altering faecal consistency
D
Innervating the myenteric plexus
Question 6
A patient with flatus, abdominal pain that is diffuse and umbilical with fatigue and constipation is diagnosed with IBS after a negative colonoscopy and endoscopy.  Which is the best first line treatment?
A
Ispaghula husk
B
Senna
C
Docusate
D
Arachis oil
Question 6 Explanation: 
Isphagula husk is a soluble fibre supplement. Other first line management for IBS with constipation include dietary changes including more soluble fibres (like oats and linseeds) and increase water intake.
Question 7
Magnesium sulphate and magnesium hydroxide are given to this patient to relieve his non-responsive constipation. What type of receptor does this rely upon?
A
Baroreceptor
B
Stretch receptor
C
Chemoreceptor
D
Thermoreceptor
Question 7 Explanation: 
These drugs are osmotic laxatives. They draw water into the gut. This stretches the walls of the GI tract, triggering peristalsis and the defecation reflex.
Question 8
A patient with faecal impaction is given an osmotic laxative that is an inert polymer of ethylene glycol. What is this name of this medication?
A
Lactulose
B
Senna
C
Macrogol
D
Saline purgative
Question 9
A patient with constipation, liver flap on general examination, generalised confusion and day-night somnolence inversion is given a medication to treat their condition. As this medication is fermented, it is converted into...
A
Acetic acid
B
Glucose
C
Lactulose
D
Ascorbic acid
Question 9 Explanation: 
The stem is describing the drug lactulose which is converted by microbiota of the patient's gut to lactic acid and acetic acid. It is readily used in chronic constipation patients and to treat hepatic encephalopathy.
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