Differential Diagnosis for Skin Conditions: SOAP Guide

The unusual feature of this assignment is that your entire objective dataset is a photograph. There is no patient to question, no palpation, no history — just a graphic and whatever you can read from it. That constraint is the assignment's point rather than its limitation: it forces you to describe before you diagnose, which is the discipline the brief names when it asks how you would describe the abnormal physical characteristics *using clinical terminologies*. Most students look at an image, recognise something, and then write a description that has been reverse-engineered from the diagnosis they already picked. Doing it in the correct order — describe fully, then generate possibilities from the description — is what the differential process actually is. A practical way to enforce that order is to write the objective section of the SOAP note in full before you allow yourself to name any condition at all, since the template's own structure then does the discipline for you.

Clinical terminology here means the vocabulary of lesion morphology, and using it precisely is the fastest way to demonstrate competence. Start with the primary lesion: is it a macule, papule, plaque, nodule, vesicle, bulla, pustule or wheal? Those terms encode size and whether the lesion is raised, fluid-filled or flat, and choosing between them is already diagnostic work. Then secondary changes: scale, crust, excoriation, lichenification, erosion, ulceration, atrophy. Then the qualifiers — colour, border definition, surface characteristics, and whether the lesion blanches. A description written in those terms is worth several paragraphs of ordinary English, because each word rules something in or out. Say how many lesions there are and how large they appear as well, because number and size are part of the primary description and both discriminate between conditions that share the same lesion type.

Configuration and distribution carry as much information as morphology and are more often skipped. Configuration is how lesions relate to each other: annular, linear, grouped, serpiginous, target-shaped, reticular. Grouped vesicles suggest something quite different from scattered ones. Distribution is where they sit — extensor versus flexural surfaces, photodistributed, dermatomal, intertriginous, acral — and it is frequently the single most discriminating feature available. Flexural involvement points one way and extensor involvement another for two conditions that can look similar close up. Say what you can see about distribution even when the graphic is cropped, and say explicitly when the framing does not let you assess it. Note whether the surrounding skin is involved too, since a lesion with a surrounding rim of erythema or scale behaves differently in a differential from an identical lesion arising on entirely normal skin.

That last point deserves emphasis because it is where honesty earns marks. You are working from an image, so there are things you genuinely cannot determine: whether the lesion is tender or itchy, how long it has been present, whether it is changing, whether there is systemic upset, and often its true size and colour under normal lighting. Rather than inventing these, state what you would need to know and why it would discriminate. The brief tells you to pretend this is an actual patient and give as much detail as possible, which some students read as licence to fabricate a history — a better reading is that you should specify what history you would take and what you would expect each answer to rule in or out. Say what the photograph itself may distort as well, because lighting and colour balance can shift the apparent hue substantially, and a diagnosis resting on a colour judgement from an image should acknowledge that.

Build the differential from the description rather than from the diagnosis you first suspected, and give each candidate a discriminating feature. If the image shows well-demarcated erythematous plaques with silvery scale on extensor surfaces, the differential includes psoriasis, but also nummular eczema, tinea corporis and cutaneous lupus — and the discriminators are scale quality, border, distribution and central clearing. Order the list by likelihood and say why the leading candidate leads. Then, crucially, address whether anything in the image warrants urgency: asymmetry, irregular borders, colour variegation, diameter and evolution are the features that make a pigmented lesion a different kind of problem, and a differential that never considers malignancy is incomplete. Say what you would do if the leading candidate turned out to be wrong too, because a differential is only useful if it has a second choice, and naming what you would reconsider is what makes the list an actual plan.

Finally, use the supplied SOAP template properly. Subjective is where the history you would take belongs, written as what you would ask and expect rather than as invented answers. Objective is your morphological description, which should be the most detailed part of the note given that it is the only real data you have. Assessment carries the differential with your leading diagnosis and the reasoning. Plan covers any diagnostic testing you would order — potassium hydroxide preparation, biopsy, dermoscopy — and the management that would follow. Support your reasoning with evidence-based sources as the brief instructs, and choose only one condition from the Skin Conditions document as it specifies. Keep the plan proportionate to the certainty you have as well, since the honest output of a single photograph is often a testing plan rather than a treatment one, and proposing definitive management from an image overstates what you know.