Explaining a suspected fungal toenail and the confirm-before-treat principle — OSCE

Candidate instructions (8-minute station)

You are the paediatric registrar in clinic. A 9-year-old boy has an 18-month history of a thickened, yellow, crumbly left great toenail with subungual hyperkeratosis and distal onycholysis. His father has tinea pedis and a thickened toenail. The family has come expecting tablets to clear it quickly. [4]

Your tasks are: [1]

1. Explain the likely diagnosis — a fungal nail infection or onychomycosis — in plain language, and how his father's foot infection is linked. [4]
2. Explain why a test (a nail sample for mycology) must come before any tablets, and what that test involves. [4]
3. Outline the treatment ladder once confirmed — topical for limited disease, weight-based oral terbinafine for extensive disease — and set the expectation about how long a toenail takes to look normal. [4]
4. Explain the prevention (treating the household contact) and name the red flags in a disordered nail that would bring the child back sooner. [1]

You are not expected to start systemic therapy in this station — the confirm-before-treat principle means the test comes first. [4]

Examiner prompt to the actor (parent)

"He has had this ugly nail for ages and it is embarrassing him at swimming. His dad has the same thing, so surely it is just fungal — can you just give him the tablets today and get rid of it? I do not want to come back for tests, it is obvious what it is. And will it spread to his other toes?" [4]

Marking domains

• Frame and explanation (3): explains onychomycosis as a fungal infection of the nail in plain, reassuring language; names that it is not dangerous and is commoner when a household contact has a fungal foot infection; acknowledges the child's embarrassment and the desire for a quick fix. [4]
• Confirm-before-treat (3): explains clearly that a nail sample (subungual debris for potassium hydroxide and fungal culture or polymerase chain reaction) must come before tablets, because the appearance overlaps with other conditions and the tablets carry a small risk to the liver; frames the test as protecting the child, not as red tape. [4]
• Treatment and timing (2): explains the ladder — a topical lacquer for limited disease and weight-based oral terbinafine for extensive disease, for about 12 weeks for a toenail — and sets the honest expectation that a normal-looking toenail is 12 to 18 months away because nails grow slowly. [4]
• Prevention and red flags (2): explains that treating his father's tinea pedis reduces reinfection and spread to other toes, and names the red flags — a brown band widening or darkening in one nail, or a nail being destroyed — that would bring the child back sooner. [1] [7]

Model answer — the explanatory script

"Thank you for bringing him in. What you are describing — a thickened, yellow, crumbly nail with debris under it — is very likely a fungal nail infection, called onychomycosis, and you are right that his dad's fungal foot infection is probably linked, because these infections pass between family members and through wet changing-room floors. It is not dangerous, and it is very treatable, but I want to get this exactly right for him." [4]

"Let me answer your questions in order — the test, the tablets, the timing, and how we stop it coming back." [4]

"First, the test. I know it looks obvious, but I would be doing him a disservice if I just gave the tablets today. Fungal nails can look almost identical to other nail problems like psoriasis or old trauma, and the tablets we use can, in a small number of children, affect the liver, so we never give them unless we have proof. The test is simple — I take a little of the crumbly debris from under the nail, from the part that is actively growing, and the lab checks for fungus under the microscope and grows it in a culture. A couple of weeks later we know for certain, and then the tablets are safe and justified. It protects him, and it is the right way to do this." [4]

"Second, the tablets. If the test confirms fungus, and given this nail has been going for eighteen months, the best treatment is a tablet called terbinafine, which I give by weight — for a boy his size that is one tablet once a day for about twelve weeks. There is also a lacquer you paint on, but that is really only for very minor nails. The tablet works well, but I do keep an eye on his liver with a blood test. And I want to be honest about timing: toenails grow slowly, only about three millimetres a month, so a brand-new normal nail is going to take twelve to eighteen months to grow all the way out. The medicine clears the fungus, but the nail itself has to grow back, and that just takes time." [4]

"Third, stopping it coming back. Because his dad has the fungal foot infection, we should treat that too — otherwise dad keeps passing it back to him, and it can spread to his other toes. So I will ask dad to see his doctor about his feet, keep his feet dry, and wear sandals at the pool. Together that gives him the best chance of clearing it for good." [4]

"And last, what to watch for. This is almost certainly a straightforward fungal nail, but I want you to come back sooner if you ever see a brown stripe down one of his nails that is getting wider or darker, or a nail that seems to be breaking down or growing a lump under it — those are the rare things we want to pick up quickly, even though they are not what is happening here." [1] [7]