Thou shalt, if all else fails or if the evidence is lacking, happily consider watchful waiting as an appropriate course of action

The first commandment assumes that there will be two diagnoses in each consultation.
But often there will be more, or none. Many consultations consist of a complex dialogue of exploration, attempted understanding, and partial uncertainty. Unless there is a clear diagnosis, it is usually better to keep the offer open of another consultation rather than issue a prescription.
Other situations where it is often better not to prescribe include acute self- limiting illnesses where symptomatic treatments are available over the counter (OTC). This also applies to some more chronic conditions such as irritable bowel syndrome and chronic back pain, which characteristically fluctuate and for which prescription-only treatments are usually no more effective than cheap OTC alternatives.
The temptation to prescribe rather than offer a timely reassessment should always be resisted.

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Fun and

Bridge Over Diagnosis - a parody of the Simon and Garfunkel classic Bridge Over Troubled Water.