12.3 Scenario Practice for Microbiology
Key Takeaways
- Watery, follicular conjunctivitis with preauricular node and recent URI points to adenovirus (viral); thick purulent discharge that glues the lids points to bacterial conjunctivitis.
- A contact-lens wearer with severe pain out of proportion, ring infiltrate, and water exposure should raise suspicion for Acanthamoeba keratitis; Pseudomonas causes a rapidly progressive ulcer with hypopyon.
- A dendritic (branching) corneal lesion staining with fluorescein is the hallmark of herpes simplex keratitis — never treat with topical steroids alone.
- Match the discharge type, time course, risk factors, and laterality in the stem to the most likely organism before choosing the action.
12.3 Scenario Practice for Microbiology
COA scenario items describe a red or painful eye and bury the diagnostic clue in the discharge type, time course, laterality, and risk factors. Read for those four cues, then pick the organism and the action.
Reading the discharge and history
| Cue in the stem | Most likely cause | Tip-off detail |
|---|---|---|
| Thick, purulent, lids stuck shut in the morning | Bacterial conjunctivitis (Staph, Strep pneumoniae, H. influenzae) | Mucopurulent discharge, rapid onset |
| Watery/serous discharge, follicles, preauricular lymph node, recent cold | Adenovirus / EKC (viral) | Started one eye, spread to the other; highly contagious |
| Itching, ropy mucus, bilateral, seasonal | Allergic (not infectious) | Distractor — not a microbiology answer |
| Branching (dendritic) ulcer staining with fluorescein, pain, decreased vision | Herpes simplex keratitis | Unilateral, recurrent; steroids alone are contraindicated |
| Contact-lens wearer, severe pain, ring infiltrate, water/shower exposure | Acanthamoeba keratitis | Pain out of proportion to findings |
| Contact-lens wearer, rapidly enlarging ulcer with hypopyon | Pseudomonas aeruginosa keratitis | Hours-to-days progression, sight-threatening |
Worked scenario 1 — the contagious red eye
A 30-year-old presents with a 3-day history of a red, watery right eye that has now involved the left eye. He had a head cold last week and has a tender node in front of the right ear. The cues — watery discharge, follicular reaction, preauricular node, recent upper-respiratory infection, second-eye spread — point to adenoviral epidemic keratoconjunctivitis. The COA action: glove, perform hand hygiene, use disposable or bleach-disinfected instruments, and disinfect the slit-lamp chinrest and headrest, because EKC outbreaks are notorious for spreading through eye clinics.
Worked scenario 2 — the contact-lens emergency
A soft-contact-lens wearer reports severe pain in one eye after swimming in a lake while wearing lenses; the exam shows a corneal ring infiltrate and pain far worse than the visible signs. Roughly 85% of Acanthamoeba keratitis occurs in contact-lens wearers, and water exposure (swimming, showering, rinsing lenses or cases in tap water) is the leading risk behavior. This is Acanthamoeba keratitis until proven otherwise; the COA must flag it urgently for the ophthalmologist and reinforce that lenses must never contact water.
The herpes trap
Whenever a stem describes a dendritic, branching corneal lesion that stains with fluorescein, the answer is herpes simplex keratitis. The classic distractor offers a topical steroid — steroids alone worsen HSV and can perforate the cornea, so the safe answer is antiviral therapy directed by the physician, never a steroid by itself.
Worked scenario 3 — the post-injection alarm
A patient who received an intravitreal injection two days ago calls with increasing pain, redness, and worsening vision. The cues — recent intraocular procedure, pain, vision drop — point to endophthalmitis, most often caused by the patient's own coagulase-negative Staphylococcus seeded at the time of the procedure. The COA does not reassure or schedule a routine visit; this is a same-day, sight-threatening emergency that must reach the ophthalmologist immediately. Any answer that delays evaluation is wrong, regardless of how mild the early symptoms sound.
Worked scenario 4 — fungal after a farm injury
A landscaper presents with a corneal ulcer with feathery, branching margins that began after a tree branch struck the eye. Vegetable-matter trauma is the classic setup for fungal keratitis (Fusarium, Aspergillus); the indolent course and feathery infiltrate distinguish it from a fast bacterial ulcer. The COA documents the mechanism of injury carefully — "organic/plant matter" — because that single history detail steers culture and antifungal therapy.
A decision shortcut
When two organisms seem plausible, weight the risk factors in the stem above the appearance:
| If the stem emphasizes | Lean toward |
|---|---|
| Recent surgery / injection + vision loss | Endophthalmitis (S. epidermidis) |
| Contact lens + water + severe pain | Acanthamoeba |
| Contact lens + rapid ulcer + hypopyon | Pseudomonas |
| Plant/vegetable trauma, feathery ulcer | Fungal |
| Recent cold + watery eye + ear node | Adenovirus (EKC) |
| Recurrent unilateral dendrite | HSV |
Driving every answer back to the risk factor — not the most familiar word in the options — is the habit that turns scenario items into reliable points.
The six-step reading method
Apply the same routine to every scenario stem so nothing is missed: (1) identify laterality — one eye or both, and did it spread; (2) read the discharge — watery, purulent, or stringy; (3) note the time course — hyperacute hours, acute days, or chronic weeks; (4) extract the risk factors — lens wear, water, trauma, surgery, recent illness; (5) name the most likely organism; (6) choose the action — culture before antibiotic, isolate and disinfect, or emergent referral. Skipping step 4 is the most common reason candidates pick the appearance-matching distractor over the correct risk-driven answer.
Practicing this six-step pass on untimed stems first, then under time, builds the reflex that the exam is really measuring.
One more discriminator: hyperacute discharge
If a stem describes a profuse, copious purulent discharge that returns within minutes of wiping it away, think hyperacute bacterial conjunctivitis (classically gonococcal Neisseria gonorrhoeae) — a true emergency that can perforate the cornea. The volume and speed of the discharge separate it from ordinary bacterial conjunctivitis, and the correct action is urgent physician evaluation, not reassurance.
A soft-contact-lens wearer reports severe eye pain after showering and swimming in lenses. The cornea shows a ring infiltrate and the pain seems far worse than the visible findings. Which organism should be suspected first?