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The Red Eye: Differential Diagnosis (MRCP Part 1)

TL;DR;

In MRCP Part 1, red eye questions test rapid clinical pattern recognition and safe triage rather than specialist ophthalmology. You must quickly distinguish benign conditions (e.g. conjunctivitis) from sight-threatening causes such as acute angle-closure glaucoma, keratitis, and uveitis using a few key clinical clues. This article provides an exam-focused framework, common traps, and a mini-MCQ to secure easy marks.


Why this matters for MRCP Part 1

“The red eye” is a high-yield presentation because it integrates ophthalmology with general medicine, rheumatology, neurology, and infectious disease. MRCP questions rarely expect fundoscopy skills; instead, they assess whether you can:

  • Identify red flags

  • Prioritise urgent referral

  • Avoid dangerous misclassification (e.g. treating glaucoma as conjunctivitis)

This makes the topic a frequent discriminator between pass and fail.

For syllabus context, see the official MRCP(UK) guidance:https://www.mrcpuk.org/mrcpuk-examinations/part-1


Exam approach: think in two steps

Step 1: Is this eye painful or is vision reduced?

  • Yes → consider sight-threatening causes

  • No → usually benign

Step 2: Is there corneal or pupillary abnormality?

  • Yes → urgent pathology

  • No → conjunctival or superficial disease

This binary framework works reliably under time pressure.

Core causes of red eye you must recognise

1. Conjunctivitis (viral, bacterial, allergic)

  • Pain: mild / gritty

  • Vision: normal

  • Discharge:

    • Purulent → bacterial

    • Watery → viral

    • Itchy, bilateral → allergic

  • Key point: pupil and cornea are normal

2. Subconjunctival haemorrhage

  • Pain: none

  • Vision: normal

  • Appearance: sharply demarcated red patch

  • Associations: coughing, hypertension, anticoagulants

  • Exam trap: dramatic appearance but benign

3. Episcleritis

  • Pain: mild ache

  • Vision: normal

  • Clue: sectoral redness

  • Associations: rheumatoid arthritis, inflammatory bowel disease

4. Scleritis

  • Pain: severe, deep, boring

  • Vision: may be reduced

  • Key distinction: pain often worse at night

  • Associations: rheumatoid arthritis, systemic vasculitis

5. Keratitis (corneal ulcer)

  • Pain: severe

  • Vision: reduced

  • Photophobia: prominent

  • Major risk factor: contact lens use

  • Red flag: corneal opacity or epithelial defect

6. Anterior uveitis (iritis)

  • Pain: moderate

  • Photophobia: marked

  • Pupil: small, irregular

  • Associations: HLA-B27 conditions (e.g. ankylosing spondylitis)

7. Acute angle-closure glaucoma

  • Pain: severe eye pain + headache

  • Vision: blurred with haloes

  • Systemic: nausea, vomiting

  • Pupil: mid-dilated, fixed

  • Classic phrase: “rock-hard eye”

8. Endophthalmitis

  • Pain: severe

  • Vision: rapidly worsening

  • Context: recent eye surgery or intravitreal injection

  • Action: immediate ophthalmology referral

MRCP Part 1 ophthalmology study setup focusing on red eye differentials

One-glance comparison table (high-yield)

Condition

Pain

Vision

Pupil

Key clue

Conjunctivitis

Mild

Normal

Normal

Discharge

Episcleritis

Mild

Normal

Normal

Sectoral redness

Scleritis

Severe

↓

Normal

Deep boring pain

Keratitis

Severe

↓

Normal

Contact lenses

Uveitis

Moderate

↓

Small/irregular

Photophobia

Acute angle-closure glaucoma

Severe

↓

Mid-dilated

Halos, vomiting

Five most tested subtopics in MRCP Part 1

  1. Conjunctivitis vs uveitis

  2. Acute angle-closure glaucoma recognition

  3. Contact lens–related keratitis

  4. Systemic disease associations (RA, HLA-B27)

  5. Urgent vs non-urgent referral decisions


Mini-MCQ (exam style)

Question A 60-year-old woman presents with sudden onset of a painful red eye, blurred vision, headache, and vomiting. Examination shows a mid-dilated pupil and corneal haze. What is the most likely diagnosis?

Answer: Acute angle-closure glaucoma

Explanation Severe pain, visual loss, systemic symptoms, and a fixed mid-dilated pupil are classic. This is a same-day ophthalmic emergency and a common MRCP Part 1 discriminator.

For more exam-standard practice, see:https://www.crackmedicine.com/qbank/


Common pitfalls (very exam-relevant)

  • Assuming all red eyes are conjunctivitis

  • Missing glaucoma when nausea is present

  • Forgetting contact lenses = keratitis until proven otherwise

  • Confusing episcleritis (benign) with scleritis (serious)

  • Ignoring pupil size and shape in the stem

Practical revision checklist

  • Memorise pain + vision + pupil as your triad

  • Learn the comparison table, not long descriptions

  • Actively scan stems for contact lens use

  • Link uveitis with HLA-B27 diseases

  • Test yourself under time pressure with mocks:https://www.crackmedicine.com/mock-tests/


FAQs

Is red eye a common MRCP Part 1 topic?

Yes. It appears frequently because it tests safe triage and clinical reasoning rather than specialist detail.

What is the single most important diagnosis not to miss?

Acute angle-closure glaucoma, due to the risk of permanent vision loss.

Do I need to know detailed ophthalmic drug doses?

No. Diagnosis and urgency are more important than pharmacological detail.

Are images used in MRCP red eye questions?

Occasionally. Focus on corneal clarity, pupil size, and distribution of redness.


Ready to start?

Consolidate this topic with timed practice. Use our Practise ophthalmology MCQs and then assess readiness with a Take a full mock exam aligned to MRCP standards.


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