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Figure 2.4 Clinical reasoning step‐by‐step: define and refine the system.

Once the problem is defined, the next step is usually to consider the system involved (Figure 2.4). For every clinical sign, there is a system(s) that must be involved, that is it ‘creates’ the clinical sign – this is what is meant by defining the system. However, the really important question is, How is it involved? This is refining the system. The key questions in this case are What system is involved in causing this clinical sign? (define the system), and Do I have a primary, that is, structural problem of a body system, or a secondary problem, that is, functional problem where the system involved is affected by other factors? (refine the system).

Key concept
Define the system	For every clinical sign, identify the system that must be involved.
Refine the system	For every defined system, determine how the system is affected – primary (structural) or secondary (functional) or, for some problems, local vs systemic.

Examples include the following:

 The body system always involved when a patient vomits is the gastrointestinal (GI) system (define the system).However, it may be directly involved due to primary pathology of the gut, such as parasites, inflammation, neoplasia and foreign body. The system is thus refined as primary (structural) GI disease.Or vomiting may be occurring due to dysfunction of non‐GI organs, such as the liver, kidney, adrenal glands and/or pancreas. The system is thus refined as secondary (functional) GI disease.

 The body system that is always involved when a patient has generalised weakness is the neurological/neuromuscular system (define the system).However, in refining the system, it may be directly involved due to primary neurological/neuromuscular pathology (e.g. inflammation, toxins, neoplasia and infection).Or the neurological/neuromuscular system may be malfunctioning due to the effect of pathology on other organs, causing metabolic derangements that impair neurological/neuromuscular function, such as hypoglycaemia, anaemia, hypoxia and electrolyte disturbances. This is thus refined as secondary neurological/neuromuscular disease.