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Q1: Recognition and mechanism (3 min)
Examiner: A young farmer is brought to casualty unconscious, frothing at the mouth and nose, with pinpoint pupils and visible muscle twitching. What is your diagnosis and what is the molecular mechanism?
Candidate: The diagnosis is an acute organophosphate (OP) cholinergic crisis — the combination of miosis, bronchorrhoea, bronchospasm, fasciculations and bradycardia with a history of pesticide exposure is pathognomonic. The molecular mechanism is that the organophosphate phosphorylates the active-site serine-203 of acetylcholinesterase (AChE) via its phosphoryl (P=O) group, forming a covalent, essentially irreversible bond. AChE activity falls, synaptic acetylcholine rises roughly 100-fold, and acetylcholine then overstimulates three receptor families — muscarinic (exocrine glands, smooth muscle, heart), nicotinic (skeletal neuromuscular junction and autonomic ganglia) and central (cortical and brainstem receptors) — producing the wet, fasciculating, encephalopathic picture and the death mechanism, which is respiratory failure from the sum of bronchorrhoea, bronchospasm, respiratory-muscle paralysis and central apnoea.
Q2: The three receptor families — distinguish them clinically (3 min)
Examiner: Walk me through the clinical effects at each of the three receptor families.
Candidate: At muscarinic receptors the picture is 'wet' — salivation, lacrimation, sweating, bronchorrhoea, bronchospasm, miosis (the most reliable bedside sign, present in over 90 percent of symptomatic cases), bradycardia, hypotension, GI cramps, diarrhoea, urination and emesis — remembered as DUMBELSS (Diarrhoea, Urination, Miosis, Bronchorrhoea/bronchospasm, Emesis, Lacrimation, Salivation, Sweating). At nicotinic receptors the effect is biphasic — initial excitation producing muscle fasciculations, then depolarisation block and paralysis including of the respiratory muscles; nicotinic ganglionic stimulation also produces tachycardia, hypertension and even mydriasis that paradoxically oppose the muscarinic bradycardia and miosis, which is a classic examiner trap. At central receptors, anxiety, confusion, ataxia, slurred speech, tremor, generalised tonic-clonic seizures, coma and central respiratory depression. The three together explain why death is from respiratory failure — bronchorrhoea and bronchospasm from muscarinic effects, paralysis from nicotinic effects, and central apnoea all operating at once.
Q3: The two antidotes — doses, end-points and timing (3 min)
Examiner: You have a severely poisoned patient in front of you. What are the two antidotes, the doses, and how do you titrate each?
Candidate: The two-antidote doctrine is atropine plus pralidoxime. Atropine is a competitive muscarinic antagonist — it dries secretions, bronchodilates and raises the heart rate. The regimen is 1.2 to 3 mg IV bolus (0.05 mg/kg in children), doubled every 5 minutes until the patient is 'dry' — dry mouth and axillae, clear chest on auscultation, HR over 80/min, SBP over 80 mmHg, pupils no longer pinpoint — then an infusion at 10 to 20 percent of the loading dose per hour, continued for days to weeks because the OP is lipid-soluble and redistributes from fat stores. The critical end-point is the dry lung and HR over 80 — NOT dilated pupils or dry flushed skin, which are atropine toxicity. Pralidoxime (2-PAM) reactivates AChE by nucleophilic attack on the phosphorylated serine — it reverses the nicotinic (muscle) effects and improves central effects, but only before the enzyme has aged. The WHO high-dose regimen is 30 mg/kg IV bolus over 15 to 30 min, then 8 mg/kg/h infusion for at least 24 to 48 h, continued until no fasciculations, clear chest and off ventilation for over 12 h. It must be given EARLY — ageing half-life is minutes for soman, hours for sarin and dimethoate.
Q4: Staff protection, intubation and seizures (2 min)
Examiner: The two nurses in the bay now have watering eyes and a headache. What went wrong, and what drug will you NOT use when you intubate this patient?
Candidate: What went wrong is secondary healthcare-worker contamination from off-gassing of the solvent and dermal contact. Atropine and pralidoxime protect the patient, NOT the staff — staff protection requires full PPE (gown, double gloves, mask with eye protection), removal and double-bagging of the patient's clothing, washing skin and hair with soap and water, and ventilation of the resuscitation room. When I intubate, I will NOT use suxamethonium — it is metabolised by plasma butyrylcholinesterase, which is depleted by the OP, so suxamethonium causes profoundly prolonged apnoea. I will use rocuronium (or vecuronium). For seizures I use lorazepam 4 mg IV repeated, which also reduces OP-induced CNS excitotoxicity; I avoid Class Ia antiarrhythmics (quinidine, procainamide, disopyramide) because they worsen OP toxicity, and use magnesium for QT prolongation.
Q5: The two delayed complications (3 min)
Examiner: The patient survives the acute crisis and is extubated on day 2. On day 3 he develops ptosis, difficulty swallowing and a falling vital capacity. What is this, and what is the other delayed complication I should know about?
Candidate: This is the intermediate syndrome of Senanayake and Karalliedde, appearing at 24 to 96 hours after exposure. The mechanism is prolonged AChE inhibition at the neuromuscular junction producing sustained endplate depolarisation, receptor downregulation and deficient ACh resynthesis. The hallmark is proximal and cranial-nerve weakness with respiratory-muscle involvement (ptosis, neck-flexor weakness, dysphagia, cranial-nerve palsies III, VI, VII, IX, X, XII, falling vital capacity) WITHOUT recurrent muscarinic features — clear lungs, no secretions. Management is supportive ventilation, often for 7 to 21 days, continued pralidoxime and atropine, aggressive chest physiotherapy and DVT prophylaxis; recovery is by de novo AChE synthesis. The second delayed complication is organophosphate-induced delayed polyneuropathy (OPIDP) at 1 to 3 weeks, a symmetrical distal sensorimotor polyneuropathy (calf cramping, paraesthesia, foot-drop, wrist-drop) caused by inhibition and ageing of a distinct enzyme, neuropathy target esterase (NTE), in axons — not all OPs cause it, only those that inhibit and age NTE. There is no specific antidote; treatment is supportive rehabilitation.
Q6: Prognosis, public health and prevention (2 min)
Examiner: Tell me about prognosis, and what is the single most effective public-health intervention.
Candidate: Mortality in well-resourced ICUs is under 5 percent, but in resource-limited rural settings it is 10 to 20 percent — the entire gap is ventilation access and time to atropine. Agent matters: dimethoate and profenofos carry roughly 20 percent mortality compared with under 1 percent for malathion. Predictors of poor outcome are large volume ingested, severe presentation, a lethal agent, delay to atropine and ventilation, low RBC AChE, age and comorbidity. The single most effective public-health intervention is the WHO 2020 call to phase out highly hazardous pesticides (HHPs) — banning monocrotophos, parathion and dimethoate would prevent tens of thousands of deaths annually in South Asia, because OP pesticide self-harm is among the commonest causes of deliberate self-harm death. At the bedside, every deliberate self-harm case requires psychiatric assessment once stable, and the case is a notifiable medicolegal event.