Symptom ≠ System

Reminder

The obvious organ is the obvious starting point —
not always the obvious answer.

The assumption Cough → lungs.
Vomiting → stomach.
Jaundice → liver.
These are reasonable first hypotheses — but they are hypotheses, not conclusions. Many common presentations originate in a completely different system.

The principle Symptoms are clues. Systems are hypotheses. Mechanism is the truth. Clinical reasoning requires tracing the mechanism — not stopping at the first plausible organ.

The risk Stopping at the obvious system is premature closure — one of the most common and consequential cognitive errors in medicine. The child with tachypnoea may have heart failure, not pneumonia.

The Analogy

"Cough is like smoke. Smoke doesn't always mean fire in the chimney — it can come from the basement, from chemicals, or from fog."

Smoke = Symptom — what you observe

The chimney = The obvious system — your first hypothesis

Where the fire is = The mechanism — the actual cause

The Framework

Where most students stop — and where they need to get to

Three levels of clinical reasoning

Symptom → System → Mechanism

Most stop here ✘ 👁

Symptom

What the patient presents with. The observable finding — what is seen, heard, or reported. The starting point, not the answer.

"Child has cough."

→ generates

Many stop here ✘ 🧩

System

The organ system that could produce this symptom. A hypothesis — not a conclusion. Multiple systems may produce the same symptom.

"Respiratory system."

→ traced to

Target level ★ 🔬

Mechanism

The pathophysiological process actually responsible. May originate in a completely different system. This is where diagnosis becomes possible.

"Pulmonary oedema from cardiac failure."

The system is the signpost — the mechanism is the destination. A clinician who stops at "respiratory" for a child with cough may miss cardiac failure, anaemia, sepsis, or metabolic acidosis — all of which can present with the same symptom through different mechanisms.

Examples

Symptom → Non-obvious system → Mechanism — why it happens

Paediatric examples — mechanism column is essential

Common symptoms — uncommon origins

Symptom	Non-Obvious System	Specific Cause	Mechanism Explained
Cough / Wheeze / Tachypnoea	Cardiac	Heart failure → pulmonary oedema	Elevated pulmonary venous pressure forces fluid into alveoli and interstitium, stimulating cough receptors and reducing lung compliance — producing tachypnoea and wheeze without primary airway disease.
Tachypnoea	Haematological	Severe anaemia	Reduced oxygen-carrying capacity drives compensatory increase in respiratory rate to maintain oxygen delivery. The lungs are structurally normal — the driver is haematological.
Tachypnoea	Metabolic	Diabetic ketoacidosis (Kussmaul breathing)	Metabolic acidosis triggers hyperventilation via central chemoreceptors as a compensatory mechanism to blow off CO₂ and raise pH. Deep, sighing respirations — not from lung pathology.
Vomiting	Neurological	Raised intracranial pressure	Pressure on the vomiting centre in the medulla oblongata triggers emesis directly — characteristically without nausea, often projectile, typically early morning. No GI pathology present.
Vomiting	Metabolic	Diabetic ketoacidosis	Ketonaemia and acidosis directly stimulate the chemoreceptor trigger zone, causing vomiting independent of any GI disease. Vomiting worsens dehydration and acidosis — a dangerous cycle.
Vomiting	Infective — non-GI	UTI / Meningitis	Systemic infection and CNS involvement activate the vomiting centre through inflammatory mediators and direct pressure. Common in young children where vomiting is a non-specific response to serious illness.
Jaundice	Haematological	Haemolysis	Accelerated red cell destruction releases unconjugated bilirubin faster than the liver can conjugate it — producing jaundice without any liver pathology. The liver is functioning; the load is excessive.
Poor feeding / Fatigue	Cardiac	Congenital heart disease	Increased work of breathing and reduced cardiac output make feeding effortful and exhausting. Sweating during feeds in an infant is a classic sign — the infant is working as hard to feed as an adult climbing stairs.

Worked Example

Child with fast breathing — three systems, three mechanisms, three question sets

Tachypnoea · Paediatrics

One symptom — three completely different causes

Presenting symptom

"Child is breathing fast."

Hypothesis A · Pulmonary

Lung Disease

Infection, inflammation, or obstruction reduces gas exchange — tachypnoea compensates for hypoxia or hypercapnia.

Fever? Cough? Duration?
Focal chest signs — crepitations, reduced air entry?
SpO₂ — falling with consolidation?
Noisy? Wheeze or stridor?

→ Pneumonia / Bronchiolitis / Asthma

Hypothesis B · Cardiac

Heart Failure

Pulmonary venous congestion from failing left heart floods alveoli — tachypnoea from reduced compliance and cough from oedema, not infection.

Poor feeding? Sweating during feeds?
Hepatomegaly? Gallop rhythm?
Bilateral basal crepitations?
Known cardiac lesion or murmur?

→ Heart failure / Congenital heart disease

Hypothesis C · Haematological / Metabolic

Anaemia / Acidosis

Reduced oxygen-carrying capacity (anaemia) or metabolic acidosis (DKA, sepsis) drives compensatory hyperventilation — lungs and heart are structurally normal.

Pallor? Conjunctival pallor?
Known diabetes or illness causing acidosis?
Haemoglobin — severely low?
Blood gas — metabolic acidosis?

→ Severe anaemia / DKA / Sepsis

The symptom is identical in all three. The discriminating questions are different for each hypothesis. This is why the symptom alone cannot direct management — only the mechanism can. Note 12 (Key Questions) provides the tools to separate these pathways at the bedside.

Cognitive Error Warning

Premature closure — stopping at the first plausible system

Premature closure is the tendency to stop searching once a plausible explanation is found. It is one of the most common and consequential cognitive errors in clinical medicine. The child with tachypnoea who is labelled "pneumonia" before cardiac and metabolic causes are considered may receive the wrong treatment — or no treatment for the actual problem.

The antidote is deliberate mechanistic thinking: always ask whether another system, through a different mechanism, could produce the same symptom. This is not about doubting your first hypothesis — it is about completing your reasoning before committing to it.

Common Student Errors

What symptom-equals-system thinking looks like

Assuming the presenting symptom defines the affected system without considering alternatives

Stopping the differential at the obvious organ — not asking which other systems could produce this

Missing cardiac causes of respiratory symptoms — heart failure in a child presenting as "chest infection"

Missing neurological causes of vomiting — raised ICP presenting as "gastroenteritis"

Not asking about the mechanism — knowing the system but not why that system is producing the symptom

Committing to the first plausible explanation before completing the mechanistic reasoning

Final Take-Home Message

"Symptoms are clues.
Systems are hypotheses.
Mechanism is the truth."

Start with what you see.
Generate multiple system hypotheses.
Trace the mechanism.
Only then — diagnose.

Symptom ≠ System Trace the mechanism Avoid premature closure

The obvious organ is the obvious starting point —not always the obvious answer.