The tall, youthful seventy year old woman wore her strikingly white hair in a tight bun. She was dressed like a Donald Fagen song – in jeans and pearls (”Maxine”, 1982).
She had an intense burning, itching sensation on the left side of her neck and occiput. Looking closely at her neck and hairline, I saw a couple of small, red papules. A few of them looked like early blisters.
I suspected herpes zoster and offered her a generic antiviral. The earlier you start it, the better your chances of avoiding long lasting pain afterward, I explained.
A week later, there were some red blotches and several scratch marks. Her burning and itching were worse.
I prescribed gabapentin and told her how to titrate herself up from 100 mg at bedtime to 300 mg three times a day.
The following week she still had red blotches and scratch marks and felt no better. I frowned.
She said “My granddaughters have head lice, so I asked my daughter to check me, but she couldn’t find any. Would you check me, just to make sure?”
I leaned close and removed my -11 diopter glasses. My focal point is about one finger length from my corneas.
It took me a while, but I found half a dozen nits, enough to be sure she had the real thing.
Didn’t I feel a little sheepish. Seventy year old woman with burning and itching scalp? Must be zoster, right? Head lice is more of a pediatric problem, right?
Wrong. I narrowed my differential diagnosis too quickly.
And, I didn’t take my glasses off the first time.
A Country Doctor Writes: 
Yes, lousy diagnostician, but hopefully it was one mistake and you learned.