Scenario · Dermatologists
A rash differential with sources
To build a rash differential, describe the morphology precisely (primary lesion, colour, scale, configuration such as annular, distribution, symptoms, duration and systemic features), then list the can't-miss causes first, the common causes next, and the first test that separates them, such as KOH microscopy of scrapings or a biopsy. A precise description is what narrows the list. AskMed, an AI medical reference for dermatologists, GPs and other doctors, nurses, pharmacists, students and researchers, answers that described pattern from journals, guidelines and references, and links every claim to its source. AskMed does not look at photos: an attached image is read for its text only, so the description has to be in words.
- Describe it in wordsLesion, colour, scale, configuration and distribution
- Can't-miss firstThen the common causes, each with its key feature
- First testKOH, dermoscopy, biopsy or serology, by cause
- No photo analysisImages are read for text only
The situation: a pattern that fits several diagnoses
Many rashes look alike at first glance, and the differential depends on details: an annular scaly plaque with central clearing could be tinea corporis, nummular eczema, granuloma annulare, annular psoriasis, subacute cutaneous lupus or the herald patch of pityriasis rosea, and a widespread maculopapular eruption could be a viral exanthem, a drug reaction, secondary syphilis or acute HIV infection. Dermatologists sort these from morphology, distribution and history, but even specialists check the less common causes, the features that separate them and the evidence on the first test. GPs and residents see these patterns more often than they see a dermatologist, and need the can't-miss causes on the list before they treat. The questions are usually specific to one pattern in one patient, they need the evidence behind each feature and test, and they sound like these:
- Annular, scaly, itchy plaques with central clearing on the trunk for three weeks: what is the differential, and what test comes first?
- Widespread maculopapular rash two weeks after starting a new drug, with fever: which severe drug reactions must be excluded?
- Annular plaques without scale on the backs of the hands: how is granuloma annulare told apart from tinea?
What goes wrong without a cited source
Without a cited source, a rash differential tends to stop at the first familiar diagnosis, and the costliest errors are in what is left off. Treating an annular rash with a topical steroid before excluding tinea can modify its appearance, a pattern called tinea incognito, and make the diagnosis harder. Missing a severe drug reaction such as drug reaction with eosinophilia and systemic symptoms, or the early signs of Stevens-Johnson syndrome, delays stopping the culprit drug. Secondary syphilis is known for imitating other rashes. A general image search or a symptom-checker app tends to return the commonest diagnosis with confidence, and a general chatbot may list causes without the distinguishing features or tests. A differential is only useful when each item carries the feature that separates it and the test that confirms it, from a source that can be checked.
- Tinea incognito. Steroid treatment before a KOH test can mask a fungal infection.
- A missed drug reaction. Severe cutaneous adverse reactions need the culprit drug stopped early.
- The great imitator. Secondary syphilis can mimic many rashes and needs serology.
How AskMed helps with a rash differential
AskMed turns a precise morphological description into a structured, cited differential. For a differential diagnosis question, AskMed is instructed to list the can't-miss causes first and then the common ones, each with its distinguishing feature and its first test, and to include every standard cause rather than only the most common. Before it writes, AskMed searches a fixed list of medical sources: PubMed and PubMed Central, journals such as NEJM, JAMA and The BMJ, the Cochrane Library, guideline bodies such as NICE and WHO, and references such as StatPearls, the MSD Manual and BMJ Best Practice. Its source list has no dermatology atlas or dermatology-specific journal, so answers lean on PubMed articles, reviews and these references. Each claim links to its source, and a follow-up such as "and in a child?" keeps the topic.
Try asking
Adult with annular, scaly, itchy plaques with central clearing on the trunk for three weeks, no systemic symptoms: what is the differential, which feature separates each cause, and what is the first test?
The answer lists the causes with their distinguishing features and first tests, such as KOH microscopy for tinea, each linked. It names treatment options without doses.
- Describe, don't upload. AskMed reads text from an image, such as a lab report, not the skin in a photo.
- Study types. Choose reviews, trials or "other studies, such as observational studies and case reports" for rare patterns.
- Treatment questions. Management answers name every guideline-recommended option, with no doses.
- AskMed works on iPhone and iPad, Android and the web, with one account on all of them.
Checklist: a cited rash differential
The checklist below turns a rash into a differential you can act on, and it starts with the description, because a precise morphological description is what a reference needs to narrow the list. Dermatology training uses the same order: the primary lesion, secondary changes such as scale or crust, colour, configuration, distribution, and then the history, including new drugs, travel, contacts and systemic symptoms. The steps use AskMed on iPhone, iPad, Android or the web for the cited differential and the evidence on the first test, and they end at the bedside, with the test and the examination that decide the diagnosis. Dermoscopy, a skin scraping or a biopsy will answer what no reference can, so plan the test before treatment where the differential includes infection or a severe drug reaction. Write the question in medical terms.
- Describe the primary lesion. Macule, papule, plaque, vesicle, pustule or nodule, with colour and size.
- Add secondary changes and configuration. Scale, crust, central clearing, annular, targetoid or linear.
- Give distribution, timing and history. Sites, duration, new drugs, travel, contacts and systemic symptoms.
- Ask AskMed for the differential. Ask for the can't-miss causes first, the separating features and the first test.
- Open the source for the test you choose. Confirm what the test shows and its limits.
- Test before you treat if infection is possible. A KOH test before a steroid avoids tinea incognito.
Limits for dermatology questions
AskMed has limits for dermatology that follow from how it works. It does not analyse photographs: an attached image is read for its text, so it cannot judge the appearance of a lesion, and the description in your question is all it has to go on. It is not a symptom checker and does not diagnose patients, and a differential from AskMed is a list to examine and test against, not a diagnosis. AskMed searches a fixed list of sources without a dermatology atlas or dermatology journal of its own, so specialist detail may come from PubMed abstracts and general references. Answers are written by an AI language model, with no human review before you see them, so open the source behind anything you act on. AskMed never gives doses of topical or systemic treatments. A rapidly spreading rash with mucosal involvement, blistering or systemic illness needs urgent assessment.
Frequently asked questions
What is the differential diagnosis of an annular rash?
Common causes include tinea corporis, nummular eczema, granuloma annulare, annular psoriasis, pityriasis rosea, urticaria and subacute cutaneous lupus; others include erythema migrans, secondary syphilis and sarcoidosis. KOH microscopy is a common first test when tinea is possible.
Can AskMed diagnose a rash from a photo?
No. AskMed reads text from attached images but does not analyse skin in photos. Describe the morphology and distribution in words instead.
Which severe causes of a maculopapular rash should not be missed?
Severe drug reactions such as DRESS and early Stevens-Johnson syndrome or toxic epidermal necrolysis, secondary syphilis, acute HIV infection, measles and, in children, Kawasaki disease. Ask AskMed for the features and first tests, with sources.
Is AskMed useful for dermatologists?
Yes, for checking less common causes, separating features, the evidence on tests and the guideline-recommended treatment options, each with a source. It is built for dermatologists and every other specialty.
Does AskMed give doses for topical treatments?
No. AskMed names treatment options and explains how they are chosen, but never gives doses, strengths to apply or schedules.