Differential Diagnosis for Neck Rash
Single Most Likely Diagnosis
- Contact Dermatitis: The itchy, raised, and well-demarcated nature of the rash, especially after a walk, suggests an allergic reaction to something in the environment. The improvement with prednisone and the presence of a "tail" at the top could indicate a spreading allergic contact dermatitis.
Other Likely Diagnoses
- Insect Bite Reaction: The initial itchy spot and subsequent growth could be due to an insect bite, especially given the outdoor activity on Saturday night. The reaction might have been exacerbated by scratching, leading to further inflammation.
- Eczema (Atopic Dermatitis): Given the patient's history of eczema flare-ups, it's possible that this is another episode, although the description of the rash being itchy but not dry is somewhat atypical.
- Irritant Contact Dermatitis: Although the patient mentions no new clothing, lotions, etc., it's still possible that an irritant caused the reaction, especially if the patient was exposed to something new during the walk.
Do Not Miss Diagnoses
- Lyme Disease: Although less likely given the absence of other symptoms like fever or headache, Lyme disease can cause a distinctive "bull's-eye" rash (erythema migrans) after a tick bite. The "tail" described could potentially be part of this pattern, and given the outdoor activity, it's crucial not to miss this diagnosis due to its implications for treatment and potential long-term effects.
- Cellulitis: While the rash is described as itchy and not painful, cellulitis can sometimes present with minimal pain, especially in its early stages. The absence of fever and other systemic symptoms does not rule out cellulitis entirely, and it's a condition that requires prompt antibiotic treatment.
Rare Diagnoses
- Pityriasis Rosea: This condition can cause a herald patch followed by a wider rash, but it typically doesn't have such a rapid progression or significant itchiness as described.
- Tinea Corporis (Ringworm): Although it can cause itchy, raised lesions, the description and rapid progression are less typical for ringworm, which usually has a more gradual onset and may exhibit central clearing.