Skin Rash OSCE - Drug Reaction

Case Overview

• Age/Sex: 76-year-old male
• Occupation: Retired accountant
• Setting: Emergency/clinic visit from home
• Chief complaint: "Rash on my skin"

Patient Script

Who I Am

I'm 76, recently retired, I live alone and used to do the household shopping and chores myself.

What Brings Me In

I woke up with a red rash that's been spreading over the last two days and it's quite itchy.

My Story

About 5 days ago my GP gave me a new antibiotic for a urine infection. I started the tablets and after about 48 hours I noticed some red spots on my chest. Over the next day they spread to my arms and legs and became quite itchy. Today, 3 days after the rash first appeared (5 days after starting the medicine), it's more widespread and I'm worried it might be getting worse. I do not have mouth sores or blisters. I have had a little fever and felt a bit tired. I have had rashes before as a child but nothing like this for decades.

My Medical Background

• Past medical history: hypertension, osteoarthritis
• Current medications: amlodipine 5 mg once daily, naproxen 250 mg as needed for knee pain, started trimethoprim-sulfamethoxazole (co-trimoxazole) 160/800 mg twice daily 5 days ago for a suspected urinary infection
• Allergies: I don’t think I have any known drug allergies
• Social: lives alone, drinks 1–2 beers most evenings, ex-smoker (quit 20 years ago), independent in activities of daily living
• Family: father had heart disease, mother had type 2 diabetes

What I Think & Worry About

• I’m worried this rash is something serious and might be from the new antibiotic.
• I’m worried it might spread to my face or mouth or make me very ill.
• I want to know if I should stop the tablets and what I should take instead for the urine infection.

If You Ask Me About Other Symptoms...

• Fever: "I felt a bit feverish last night, maybe a mild fever."
• Breathing: "No shortness of breath or wheeze."
• Mouth/throat: "No ulcers, no sore throat, I can eat and swallow okay."
• Blisters/peeling: "No blisters and nothing is coming off the skin."
• Joint pain: "My knees ache as usual from arthritis — nothing new."
• New exposures: "I did change laundry detergent a couple of weeks ago and I had some sun last weekend, but the spots started after the new tablets."
• Urinary symptoms: "I had some burning when I passed urine and went to the doctor for it before the tablets were started."

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Clinical Summary

Examination

• General: alert, mildly uncomfortable with pruritus
• Vitals: Temperature 37.9°C, Heart rate 88/min, Blood pressure 138/78 mmHg, RR 16/min, SpO2 97% on room air
• Skin: widespread erythematous maculopapular rash involving trunk, proximal and distal limbs; predominantly symmetric; blanching on pressure; no target lesions; no mucosal involvement (oral and conjunctival mucosa intact); no epidermal detachment or blistering
• Lymph nodes: small, mobile, non-tender cervical and axillary nodes
• Cardiorespiratory/Abdominal/Neuro: unremarkable

Investigations

• CBC: WCC 9.8 x10^9/L (4.0–11.0) (mild lymphocytes normal), Eosinophils 0.9 x10^9/L (0.0–0.5) (mild eosinophilia)
• CRP: 18 mg/L (0–5) (mildly elevated)
• LFTs: ALT 38 U/L (7–56), AST 30 U/L (10–40)
• Renal function: Creatinine 92 µmol/L (60–110), eGFR 62 mL/min/1.73m2
• Urinalysis: positive nitrites and leukocyte esterase (consistent with bacteriuria/UTI)
• Urine culture: pending (send for microbiology)

Diagnosis

Primary diagnosis: Drug-induced maculopapular exanthem (likely antibiotic-related hypersensitivity to trimethoprim-sulfamethoxazole)

• Evidence: temporal relationship with new antibiotic (rash began ~48 hours after starting), widespread pruritic maculopapular rash, mild eosinophilia, absence of mucosal involvement or epidermal detachment.

Differential diagnoses:

• Viral exanthem — less likely given the clear temporal relationship with drug and age of patient.
• Allergic contact dermatitis to new detergent — timing (detergent changed weeks earlier) and widespread symmetric distribution less typical.
• Erythema multiforme or early SJS/TEN — less likely because there are no target lesions, no mucosal involvement, no blistering or skin sloughing.
• Fixed drug eruption — usually localized and recurs at same site, not widespread.

Management

• Immediate: stop suspected offending agent (trimethoprim-sulfamethoxazole) and document a probable antibiotic allergy in the chart and advise the patient to avoid it in future.
• Symptomatic treatment: oral antihistamine (e.g., cetirizine 10 mg daily or chlorphenamine short-term) for itch; emollients and mild topical corticosteroid for symptomatic relief.
• Monitor: observe for progression (development of fever, mucosal involvement, blistering, skin pain, hypotension or respiratory symptoms). Admit if any signs of severe cutaneous adverse reaction.
• Investigations/follow-up: review urine culture to guide an alternative antibiotic (consider nitrofurantoin if appropriate and renal function allows); arrange GP or dermatology follow-up within 48–72 hours and earlier if worse.
• Patient education: advise to return immediately for any worsening rash, mouth ulcers, difficulty breathing, facial swelling, or skin peeling; provide written allergy information and consider giving an alert card.

Key Learning Points

• A morbilliform (maculopapular) rash that develops within days of starting an antibiotic is most likely a drug eruption; temporal relationship is key.
• Assess severity carefully: absence of mucosal involvement, blisters, epidermal detachment or systemic instability suggests a non-life-threatening exanthem, but close monitoring is required for progression to severe cutaneous adverse reactions.
• Management includes immediate cessation of the offending drug, symptomatic treatment, documentation of the allergy, and arranging appropriate alternative therapy guided by cultures and renal function.