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PH6.1-5 | Gastrointestinal Pharmacology — PBL Case

CLINICAL SETTING

Meena, a 19-year-old first-year MBBS student from Tirupur, collapses in the college hostel bathroom after 18 hours of profuse watery vomiting and diarrhoea following a college food-fest. She had eaten rice, dal, and fish curry at the event — approximately 60 students ate from the same batch, and 14 of them are now ill. Meena vomited 10 times and passed watery (non-bloody, non-mucoid) stools 12 times. She did not eat anything from the next batch of food and had no fever. Her roommate helps her to the college medical centre.

Trigger 1: Collapse in the Hostel

On examination at the medical centre: Meena is conscious but drowsy. Pulse 112/min, BP 90/60 mmHg, temperature 37.1°C. Skin turgor is reduced; mucous membranes are dry; she has sunken eyes. She says she feels too weak to sit up. There is no abdominal tenderness on palpation, no guarding. Her stools have been watery and pale — no blood, no mucus. She has no fever. She last urinated 10 hours ago.

DISCUSSION POINTS

  • Based on the stool characteristics (watery, non-bloody, afebrile) and clinical findings, is this most likely secretory or invasive diarrhoea? What are the implications for drug treatment?
  • Classify the severity of Meena's dehydration using WHO criteria. What clinical signs inform this classification?
  • What is your IMMEDIATE management priority before any diagnostic tests — and why?
Click to reveal Trigger 2: 12 Hours Later — The Vomiting Continues (discuss previous trigger first!)

Trigger 2: 12 Hours Later — The Vomiting Continues

Meena is given WHO ORS and IV fluids (Ringer's Lactate 30 mL/kg over 30 min for initial resuscitation). Her blood pressure improves to 104/70 mmHg and she is now alert. However, she continues to vomit every 30–40 minutes, preventing adequate oral intake. The college doctor needs to add an antiemetic. Stool microscopy from a sample collected on admission: no red blood cells, no pus cells, no trophozoites, numerous bacteria. Blood culture ordered. A rapid rotavirus antigen test is available.

DISCUSSION POINTS

  • Meena's vomiting is now the primary barrier to oral rehydration. Which antiemetic class would you choose for vomiting in acute gastroenteritis — and which receptor pathway does it target?
  • Between metoclopramide and ondansetron for this context, which is preferred and why? Consider the receptor mechanism and the clinical setting (a 19-year-old, no chemotherapy).
  • What does the stool microscopy result tell you about the likely pathogen and type of diarrhoea? Does this change your antibiotic plan?
Click to reveal Trigger 3: Day 3 — Recovery, but a New Complication (discuss previous trigger first!)

Trigger 3: Day 3 — Recovery, but a New Complication

Meena recovers with IV fluids, antiemetics, and ORS. She is discharged on Day 2. Three weeks later, she returns to the medical centre — this time with a 4-week history of intermittent crampy lower abdominal pain, bloating, and alternating constipation and diarrhoea. There is no blood in her stools. She passes 4–6 stools/day during diarrhoeal phases, with urgency and relief of pain after defecation. Colonoscopy performed: completely normal mucosa. Biopsy — normal histology. Rome IV criteria met. She asks: 'Doctor, do I have the same disease as Crohn's?'

DISCUSSION POINTS

  • How would you explain the difference between Meena's current diagnosis and Crohn's disease to her — specifically in terms of mucosal inflammation and what this means for drug treatment?
  • What pharmacological options are available for Meena's condition (post-infectious IBS)? Which drugs target motility and which target visceral hypersensitivity?
  • Meena's classmate suggests she take mesalazine (a drug he read about for 'gut inflammation'). Is mesalazine appropriate here? Explain why or why not, using the mechanism of action.

Group Task Assignments

Group 1: Dehydration assessment and fluid therapy

  • Classify Meena's dehydration using WHO Integrated Management of Childhood Illness (IMCI) criteria
  • Calculate her ORS deficit and replacement rate for the first 4 hours
  • Explain why Ringer's Lactate is preferred over isotonic saline for initial resuscitation in secretory diarrhoea

Competencies: PH6.3

Group 2: Antiemetic pharmacology and receptor mapping

  • Create a receptor-mechanism table for: ondansetron, metoclopramide, domperidone, promethazine, and aprepitant
  • Identify which receptor pathway is dominant in acute gastroenteritis-related vomiting
  • Justify the antiemetic choice for Meena using pharmacodynamic reasoning

Competencies: PH6.2

Group 3: Secretory vs invasive diarrhoea — drug safety rules

  • Compare the pathophysiology, stool characteristics, and fever pattern of secretory (cholera-like) vs invasive (dysenteric) diarrhoea
  • Explain why loperamide is contraindicated in invasive diarrhoea — cite the mechanism and a real-world consequence
  • Create a decision algorithm: 'watery/non-bloody diarrhoea' vs 'bloody/febrile diarrhoea' — which drugs are safe and which are dangerous in each?

Competencies: PH6.3

Group 4: IBD vs IBS — pharmacological implications

  • Compare IBD (Crohn's disease, ulcerative colitis) and IBS using the 'organic inflammation vs functional disorder' framework
  • List the pharmacological agents used in IBD (aminosalicylates, corticosteroids, immunomodulators, biologics) and explain why NONE of them are appropriate for IBS
  • Identify 2–3 pharmacological agents that ARE appropriate for IBS-D (post-infectious) and explain their mechanism

Competencies: PH6.5

Group 5: ORS — the pharmacology of a public health miracle

  • Explain the SGLT1 mechanism that makes ORS effective even during cholera-toxin–driven secretory diarrhoea
  • Compare the old WHO ORS formula (311 mOsm/L) with the new low-osmolality formula (245 mOsm/L) — what changed and why?
  • Identify a clinical scenario where ORS is contraindicated (ileus/bowel obstruction) and why

Competencies: PH6.3

Learning Issues

Research these questions and bring your findings to the discussion.

  1. [PH6.2] What is the mechanism of action of the major antiemetic classes (5-HT₃ antagonists, D₂ antagonists, NK₁ antagonists, H₁/M₁ antagonists), and which is most appropriate for vomiting due to acute gastroenteritis? How does the choice differ for chemotherapy-induced, motion sickness, and pregnancy-related vomiting?
  2. [PH6.3] How does oral rehydration solution work at the molecular level (SGLT1 cotransport), and why does it remain effective even when cholera toxin has inactivated the NHE3 exchanger? What are the WHO indications for IV fluid vs ORS in acute diarrhoea?
  3. [PH6.3] What is the mechanism of loperamide, and why is it contraindicated in invasive (dysenteric) diarrhoea? Cite the specific pathophysiological danger and the clinical scenario where this error has caused patient harm.
  4. [PH6.5] What is the defining histological distinction between IBD (Crohn's disease / ulcerative colitis) and IBS? How does this distinction determine the pharmacological approach — and why would mesalazine or corticosteroids be harmful rather than helpful in IBS?