The Challenge: Can you figure out why a middle-aged man is plagued by a blistering rash on his hands?

Every month, the Diagnosis column of The New York Times Magazine asks Well readers to try to solve a real-life medical mystery. Below you will find the story of a 63-year-old contractor who is annoyed by recurrent blisters that appear on his hands and fingers. It was a minor problem — distressing, rather than dangerous, the patient thought. But was it?

Below I provide the information available to the doctors who made the diagnosis. The first reader to offer the correct diagnosis gets a signed copy of my book, “Every Patient Tells a Story,” and the satisfaction of solving a really cool case.

The Patient’s Last Minute Complaint

“One more thing,” the 63-year-old man said. His doctor had her hand on the door, ready to step out. With a quick glance at the clock the young woman turned back to face her patient. He held out his hands. “I’ve got these blisters.”

It was the first time Dr. Janelle Duah had met this patient. She was an intern, a doctor in her first year of training at the Center of Excellence at the Veterans Affairs Hospital in West Haven, Conn. He was a Vietnam veteran who’d been going there for decades. She had inherited him from a trainee who had just finished her three years of postgraduate training and moved on.

Dr. Duah had a half an hour for each of her patients, and she knew she’d already spent most of that time going over the labs he’d gotten after his last visit and talking about the benefits of improving his diet and getting more exercise.

Still, she looked at the man’s hands. She didn’t see any blisters, but the skin there certainly didn’t look normal. The back of his hands had several patches of skin that were red and scarred. Near his wrist he had a couple of faint, quarter-size marks of a purplish hue. Around his knuckles the skin was irregular and shiny and dotted with small healing scabs. They were all in different stages of healing — some discrete and dark; others with only the white scaly remnant covering a barely visible lesion on a red, shiny base.

Embarrasing and Annoying Blisters

He’d been getting these blisters for years, he told her. Drove him crazy. They were unpredictable, appearing out of nowhere and at random, it seemed. He’d just wake up in the morning and look at his hands, and a couple of times a week or over the course of a month, he’d see a new blister or two, or three.

They didn’t hurt or itch — unless he forgot and put his hand in his pocket or did just about anything else, then the thin covering of the blister would break and he’d have a real mess on his hands. The blisters were filled with this clear liquid that was kind of sticky, and it would get everywhere, leaving little marks on the opening of his pants pockets. It was embarrassing. And annoying. And the sores they left on his hands were painful.

The blisters themselves were pretty small. Maybe the size of a little Phillips-head screw, he told the young doctor — the size of the scabs he had now. Sometimes they appeared in a line, but mostly they looked like little drops of water. Years ago he got tired of having them pop whenever he forgot about them. So now he’d usually just open them himself, with a pin. Then they’d heal on their own.

Hands of an Outdoorsman

The man’s hands were large with thick muscular fingers, heavily callused on the palm side and well tanned on the other — consistent with a life lived out of doors. He was a builder and spent a lot of time outside when on the job, though he hadn’t been working much that fall. Even when he wasn’t working he spent lots of time in his yard. And he and his wife frequently went camping for fun. Indeed, they were going to go camping the next day.

His nails were clipped short and his cuticles were dry but trimmed. The doctor turned his hands over; there were no blisters or scars on his palms. She moved his sleeve up as high as she could. No scars or blisters were visible on his wrist or arms.

No, he never got them any place else, just his hands, he told her.

An Allergic Reaction?

To Dr. Duah, the rash on the back of the man’s hands looked like a kind of contact dermatitis, an allergic reaction to something he’d been exposed to.

He’d been working in the garden. Had he seen any poison ivy?, the doctor asked. No, he told her. There were other plants that could cause rashes in sunshine, but she couldn’t remember which ones. He’d never been allergic to anything — not to poison ivy, not to anything.

Any new soaps, detergents or creams? Any recent chemical exposures? No, and no. No new medications.

The only real pattern he could see was that the blistering was maybe worse in the summer, but they could come at any time of year. He had his first outbreak maybe 20 years ago, but most of the time back then it was just an occasional annoyance. A few blisters a year at the most. But starting a couple of years ago, they got a lot worse, and now he had at least a few blisters most of the time.

Years of Rash

Dr. Duah wasn’t sure what this was. If it wasn’t an allergy, there were some autoimmune diseases that could cause blisters like this. But she wasn’t sure what those blisters looked like, or if they could be limited just to the hands. She was curious to see what the supervising doctor, Dr. Kathleen White, would have to say about this guy’s hands.

She laid out the patient’s case for Dr. White. The patient had a few medical problems: a little high blood pressure, he was a little overweight, and he had sleep apnea. His cholesterol was also a little high, something they’d discussed that day. But over all, this was a pretty healthy guy. Who had a rash. For years.

Dr. White examined the man’s hands closely. It did look a bit like a contact dermatitis, she agreed. Should we send him to the dermatologist?, Dr. Duah asked.

Dr. White took a good long look at the patient. Sure, she said. We’ll refer him to derm. But why don’t we try to figure out what he has before they do.

You can read Dr. Duah’s note here.

Solving the Mystery

The patient left the office, and Dr. Duah went in to see her next patient. She was going to be behind on appointments for the rest of the day.

Halfway through that appointment, there was a knock at the door. Dr. White apologized for interrupting, then handed the resident a note. I think I figured out your patient’s rash, the note read.

What did Dr. White figure out? And — extra credit — why did this patient develop this problem?

If you have any ideas, post them in the comments section. The first person to identify the correct diagnosis is the winner..