July 29, 2025

Most Missed Question in Family Medicine This Week

By Challenger Corporation · 4 minute read

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What is the most common cause of allergic contact dermatitis?

Answers:

nickel
mercury
clothing dyes
poison ivy
latex

This question is missed 41% of the time by FM residents, an amazing rate for such a simple question. And the answer is #1 - nickel. Boring but true. 🙂

The common misses on the question are all over the place. We’re suspicious that students overcomplicate the question stem, thinking about all the causes of contact dermatitis - which are legion. Or possibly allergens encountered in healthcare settings, which would be “gloves” (rubber accelerators).

This is exactly the sort of question you will encounter on exams - so try to read question stems as-is, without expanding them mentally.

Why This Question Is Often Missed

– Focusing on plant‑related allergies

Many students immediately think of poison ivy because it’s a classic type IV hypersensitivity, overlooking that nickel exposure is far more ubiquitous.

– Conflating latex and contact dermatitis

Latex allergy is often taught early but usually manifests as an IgE‑mediated reaction rather than classic allergic contact dermatitis.

What the Distractors Indicate

Option	What It Tests / Implies	Why It’s Wrong Here
Mercury	Heavy‑metal contact allergy awareness	Mercury allergy is very rare compared to nickel.
Clothing dyes	Knowledge of textile/dye allergens	Disperse‑dye hypersensitivity exists but is far less common.
Poison ivy	Classic plant‑induced contact dermatitis	Poison ivy (urushiol) is common but not as prevalent as nickel exposures.
Latex	Allergy related to rubber products	Latex reactions are typically immediate (type I), not the delayed type IV seen in ACD.

High-Yield Pearl

Nickel is the single most common cause of allergic contact dermatitis worldwide due to its ubiquitous presence in jewelry, coins, and metal fasteners.

Core Learning Objectives

Recognize the most prevalent allergens causing type IV (delayed) hypersensitivity in clinical practice.
Distinguish the clinical and immunologic differences between allergic contact dermatitis and other skin hypersensitivity reactions.

The “Test Trick” at Play

This question exploits availability bias: students recall dramatic or “textbook” examples (poison ivy, latex) and overlook the far more common, everyday culprit—nickel.

Additional Practice Questions and Remediation

Question 1

A 25‑year‑old woman presents with a pruritic rash on her earlobes that began 2 days after she switched to a new pair of inexpensive earrings. What is the most likely allergen?

A. Nickel

B. Formaldehyde

C. Paraben preservatives

D. Fragrance mix

Answer and Remediation

If you chose A (Nickel): Correct response! Nickel in costume jewelry is the most common ACD allergen.
If you chose B (Formaldehyde): Review: Formaldehyde can cause ACD in hair‑straightening products but is far less common on earlobes.
If you chose C (Paraben preservatives): Review: Parabens rarely cause ACD and are more associated with irritant reactions.
If you chose D (Fragrance mix): Review: Fragrances are common overall but still less frequent than nickel in jewelry.

Question 2

A dental assistant develops itching and redness on her hands after wearing powdered latex gloves all day. Patch testing reveals a positive reaction to thiuram mix. What type of exposure most likely caused her dermatitis?

A. Thimerosal in vaccines

B. Rubber accelerator in glove manufacturing

C. Formalin in dental impressions

D. Paraphenylenediamine in gloves

Answer and Remediation

If you chose B (Rubber accelerator): Correct response! Thiurams are rubber accelerators used in glove manufacture.
If you chose A (Thimerosal): Review: Thimerosal is a mercury preservative, rare in causing ACD.
If you chose C (Formalin): Review: Formalin can irritate mucosa but is not a common hand‑dermatitis allergen.
If you chose D (Paraphenylenediamine): Review: PPD is in hair dyes, not in glove production.

Question 3

A 40‑year‑old gardener presents with vesicular hand dermatitis after clearing ivy in her yard. Which agent is responsible?

A. Nickel

B. Urushiol

C. Chromium in leather gloves

D. Benzocaine in topical creams

Answer and Remediation

If you chose B (Urushiol): Correct response! Poison ivy’s urushiol causes classic ACD.
If you chose A (Nickel): Review: Nickel exposure is unlikely confined to gardening tools.
If you chose C (Chromium): Review: Leather tanning agents (chromium) can cause ACD but less commonly in gardening.
If you chose D (Benzocaine): Review: Benzocaine can cause contact allergy but is topical, not tool‑related.

Question 4

After switching to a new black sock brand, a patient develops an itchy, scaly rash on his ankles. Patch test is positive for disperse blue 106. What is the allergen class?

A. Clothing dye

B. Plasticizers

C. Formaldehyde‑releasers

D. Acrylates

Answer and Remediation

If you chose A (Clothing dye): Correct response! Disperse dyes are textile allergens.
If you chose B (Plasticizers): Review: Plasticizers (e.g., phthalates) are uncommon causes of sock dermatitis.
If you chose C (Formaldehyde‑releasers): Review: Some textiles use formaldehyde resins but disperse dyes are more specific.
If you chose D (Acrylates): Review: Acrylates are in glues and nail products, not textiles.

Question 5

A 30‑year‑old jeweler presents with pruritic papules on his fingertips after working with watch bands. Which patch test series is most appropriate?

A. Fragrance series

B. Metal (baseline) series

C. Rubber glove series

D. Photopatch series

Answer and Remediation

If you chose B (Metal series): Correct response! Baseline metal series includes nickel, cobalt, chromium.
If you chose A (Fragrance series): Review: Fragrances are irrelevant to metalwork.
If you chose C (Rubber glove series): Review: Rubber accelerators matter for glove users, not jewelers.
If you chose D (Photopatch series): Review: Photopatch tests for photoallergies, not relevant here.

Mini Case Discussion Prompt

Compare a patient with chronic peri‑umbilical dermatitis from nickel‐containing belt buckles to one with acute vesicular hand dermatitis from poison ivy. What differences in history, timing, lesion morphology, and management would you expect?

Frequently Asked Questions (FAQs)

What are the top causes of Allergic Contact Dermatitis (ACD)?

Nickel sulfate – Most common overall, especially in women (jewelry, piercings, jean buttons, zippers, coins).
Fragrance mix – Found in personal care products, deodorants, lotions, etc.
Preservatives
- Methylisothiazolinone / Methylchloroisothiazolinone (e.g., in shampoos, wipes)
- Formaldehyde and formaldehyde-releasing preservatives
Neomycin sulfate – Topical antibiotic in over-the-counter creams.
Balsam of Peru – A natural resin found in flavorings and fragrances.
Cobalt chloride – Often co-sensitizes with nickel; found in metal tools and dyes.
p-Phenylenediamine (PPD) – Hair dyes, temporary tattoos.
Rubber accelerators – In gloves, shoes, and elastic materials.

What are the most dangerous or difficult causes of Allergic Contact Dermatitis (ACD)?

Allergen	Severity / Challenge	Key Context
PPD	Very severe, blistering	Hair dye, black henna tattoos
MI / MCI	Widespread, hard to avoid	Baby wipes, cosmetics
Formaldehyde	Systemic, hidden exposures	Disinfectants, clothing
Neomycin / Bacitracin	Delayed wound healing, common use	OTC ointments
Rubber accelerators	Chronic hand eczema, occupational	Gloves, industrial settings
Balsam of Peru	Cross-reactive, systemic symptoms	Fragrances, food flavorings
Fragrance mix	Common, often hidden or unlabeled	Personal care products

This question appears in Med-Challenger Family Medicine Review with CME

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