Making Rash Decisions: How Dermatologists Think

This blog post is based on a webinar hosted by David Harker, MD. For upcoming webinars, click here.

Imagine a medicine resident shrugging and saying, “All EKGs look the same to me.” While this sounds ridiculous, the phrase “all rashes look the same to me” is frequently uttered by non-dermatologist clinicians at all levels of experience.

But to the trained eye, all rashes don’t look the same. Despite this, learning to observe and describe a rash gets little emphasis in general medical education. Consequently, it is a skill few clinicians outside of dermatology acquire.

This dearth of dermatology exposure in general medical education, combined with the relatively small number of dermatologists, leads to an unfortunate reality. While skin disease is very common, those adept at diagnosing skin disease are not.

What follows here is meant to provide insight into how dermatologists think and arrive at specific diagnoses. In medical school, the history is emphasized as an all-important tool and the first step of any patient encounter. While history still matters in dermatology, for most skin disease it is secondary to the physical exam in discerning the diagnosis. This reality is why learning how to observe the salient features of a rash and describe them is so critical. If you’ve correctly described a rash then you’ve identified the morphologic features that allow you to build a differential diagnosis. A rash of scaly papules and plaques has a completely different differential than a rash of vesicles and erosions. So, let’s start with some definitions you probably were exposed to briefly during your medical training.

The Basics

Primary Lesion Type

There are a few “primary lesion” types in dermatology, which refer to the different ways pathology in the skin manifests clinically. A primary lesion is sort of like what a rash boils down to. What is the essential clinically observable lesion? Bear in mind, one rash can have multiple primary lesions.

Papules, plaques, and nodules:

Papules and plaques are palpable changes in the skin—either elevated or depressed. The usual cutoff is ~1 cm in size (>1 cm is a plaque). A nodule is generally considered a palpable subcutaneous lesion.

Macules and patches:

Flat lesions that you couldn’t find by feel if your eyes were closed. Usually refers to pigmentary changes in the skin. Again, the cutoff is 1 cm (patches are >1 cm).

Bullae, vesicles, and pustules

Blisters in the skin.

  • Bulla: >1 cm
  • Vesicle: <1 cm
  • Pustule: blister filled with pus

A tense bulla is caused by subepidermal pathology (at the dermo-epidermal junction). Bullae that are easily ruptured (called flaccid bullae) are cause by intraepidermal pathology. Sometimes, in diseases like pemphigus vulgaris that cause flaccid bullae, there may not be any intact bullae on the skin—just erosions.

Erosions and ulcers

  • Defects in the skin
  • An erosion is a defect in skin involving part or all of the epidermis, but the dermis is preserved.
  • An ulcer arises from loss of the entire epidermis and part or all of the dermis.

Secondary Features

After you have determined the primary lesion(s), identifying the secondary features can help in making a diagnosis.


Retained keratin/stratum corneum on the skin. There are different types of scale, each suggesting different diagnoses. When scale is seen, the pathology is generally in the epidermis. Examples include:

  • Flaky or “bran-like” such as pityriasis rosea
  • Micaceous scale of psoriasis, meaning it resembles mica that comes off in thin layers

Erythema vs purpura

Erythema has become synonymous with “red,” when in actuality there are various shades of erythema each with their own connotations. To a dermatologist, erythematous communicates that the rash blanches with pressure. In contrast, purpura indicates a rash that does not blanch with pressure and is generally a darker purple color. This indicates extravasation of red blood cells into the dermis, which can be from many etiologies including vasculitis.

It’s important to note that erythema and purpura look different on skin of color. It can be more subtle or dusky in dark skin. There has been a lack of skin of color representation in disease images for medical education for many years, but there are ongoing efforts by many to improve this.


How are these lesions grouped? What is the relationship to each other? Is there a pattern?

Herpes simplex virus can manifest in many ways on any cutaneous surface—not just on classic locations like lips or genitals. One clue is often the classic morphology of grouped vesicles clumped together on an erythematous base. Consequently, this arrangement is called herpetiform.

Lichen nitidus: This image shows linear configuration of flat-topped papules. Koebnerization refers to lesions that have formed at sites of trauma, such as scratching.

Erythema annulare centrifugum: Here the annular, or ring-like, lesions coalesce into a lager plaque. The scale of erythema annular centrifugum follows behind the edge of lesion. This is in contrast to tinea, in which scale usually leads the edge of lesion.

Other global patterns / arrangements of lesions:

Reaction Patterns

After you’ve identified the primary lesion(s), salient secondary features, and noted lesion arrangement, etc., rashes can usually be categorized into what dermatologists call reaction patterns. These are categories of dermatologic disease morphology that suggest specific differential diagnoses.

At right: An example of a vesiculobullous reaction pattern: erosions, vesicles, and bullae on skin (eg, pemphigus vulgaris).

A Moose is a Moose

To the trained eye, some rashes have such distinct presentations that the diagnosis can be made “from the door.” These rashes are kind of like a moose. You only have to see a moose once to know it’s a moose and to easily identify another moose in the future. Examples of these include classic discoid lupus, psoriasis, or a basal cell carcinoma. But sometimes all a dermatologist can do is place a rash into a reaction pattern, generating a specific differential based on the physical exam and history, and then must rely on histology to help further tease out the final diagnosis.


The skin is an easy organ to sample and skin biopsies help dermatologists make diagnoses every day. But the histologic findings on a biopsy as a single data point is often insufficient to make a correct diagnosis. We must integrate clinical findings on the exam with what is found under the microscope. Dermatologists learn a lot of pathology in residency which facilitates discussing difficult cases with dermatopathologists to clinch a diagnosis.

About Dr. Harker

Dr. Harker earned his doctor of medicine degree at The University of Texas Southwestern Medical School. In addition to excelling in his studies and clinical rotations, Dr. Harker invested significant time researching the immunology of melanoma, presenting posters at academic conferences, and contributing to publications in high-profile dermatology journals. After medical school, Dr. Harker completed an internship in internal medicine at Baylor University Medical Center. He then returned to The University of Texas Southwestern Medical Center for dermatology residency training. During residency, Dr. Harker served as chief resident over the VA dermatology clinic. As chief resident, he provided critical leadership through the COVID-19 crisis and was instrumental in establishing a teledermatology service for the clinic. Dr. Harker was recognized for his clinical acumen and surgical skills as a resident. In July 2020, Dr. Harker joined the Skin Surgery Center in Hickory, North Carolina.

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