"I think it will be safe to leave your X-rays here," Victor McKusick says, gesturing to a nearby table, to a medical student dressed in whites. The student, who promptly obliges, is one of seven clustered in the fourth-floor office in the Billings Administration Building.

McKusick, University Professor of Medical Genetics, stands in front of a utility closet-looking door and scans the group. The four women are wearing shorts; the three men sport pants. They're a touristy-looking bunch, most with cameras in their hands or on a string around their neck. McKusick motions to one of them. "That heavy bag might be a problem."

The students, eager and responsive, quickly get the message: Carry light.

From here it's upwards and onwards, right to the very top of the Hopkins dome.

McKusick, known by many as the father of medical genetics, is a renowned figure at Johns Hopkins, with more than 50 years of research, teaching and clinical experience at the very school where he received his medical degree. But a few days each year, McKusick wears a different hat, that of tour guide to JHMI's quintessential symbol.

He unlocks and opens the door, beckoning, "Here we go then. Watch your step."

Slowly and in single file, the group plods up the first flight of bare wooden steps. Loud, steady clomps break the quiet.

The students, classmates in a basic radiology elective, learned of McKusick's dome tours through their instructor, Donna Magid, an associate professor of radiology and orthopedic surgery. Magid annually sends her students out with McKusick, and enrollment in the voluntary activity is typically 100 percent.

At the top of the first flight of stairs is a dimly lit, wood-floored and -paneled room. Piled neatly against the walls are stacks of stuffed manilla folders, filing cabinets and assorted boxes. A small sign on the wall reads "Attic."

No sight of the dome yet. McKusick disappears around the corner on the far side of the room. When the students catch up, several of their mouths drop at the sight of McKusick ascending a wall on metal rungs.

"You've got to be kidding me," one female student blurts out, smiling. "No wonder they said don't wear a skirt."

As others turn the corner, they chime in with "wow," "holy mackerel" and "this is so cool."

Without trouble, each student scales the rungs. On solid flooring again, the entire group is now literally inside the Hopkins dome. The air is warm and stagnant, ripe with aging wood, like an old barn. The inside of the dome is vast and open, and many stop to take in the spectacle. Signs of feathered guests are scattered on the floor, making visitors feel like they're standing in the bottom of a huge bird cage.

The next hurdle is a tall, steep flight of steps that snakes around the dome's perimeter. They march onward; their prize still awaits.

The domed administration building dates from 1889 and is named for John Shaw Billings, the architect/physician/librarian who designed the original Johns Hopkins Hospital.

Markings of dome visitors who have come before are scrawled on the wall at the next landing. From top to bottom, the wall is full of names, matched with either the date of their visit or their graduating class.

More stairs have yet to be climbed, but McKusick assures the group that this is the last leg of the journey.

These last stairs end at a padlocked trap door in the ceiling. McKusick reaches for his keys, and everyone can just taste the view only a few feet away.

Or so they hoped.

"It would appear they have changed the padlock, or that this is a bad key," McKusick says calmly. "We're going to have to come down."

A chorus of groans burst out. The disappointment is palpable.

"We've come so far," Sharon Chung says. "Someone must have a key."

Equipped with a cell phone, another student offers to call someone who could help. McKusick tells him to go ahead.

"Hi Marge, it's Jon Dorfman calling, from Dr. Magid's class. We are here with Dr. McKusick, and we are trying to go up to the top of the dome," Dorfman says, "but they have changed the lock and he doesn't have the right key."

"For the padlock," someone interjects.

"Yes, the padlock key," Dorfman says.

Success. There is another key, but it has to be picked up some 150 feet below, by the Jesus statue in the building's rotunda. McKusick tells the group to head down to the fourth-floor office where the trip started.

Once there, McKusick asks the students if they want to call it a day, or wait for the key. Unanimously, they agree to wait while one of them retrieves the key. Several minutes later, it's time to retrace their steps.

"How many times have you been up here Dr. McKusick?" asks Tonya Kuhn.

"Oh, about 150," he says, making it sound like a conservative estimate.

McKusick's first trip to the top of the dome was back in 1943, during his days as a medical student. Back then the attic door was unlocked, McKusick says, and he and others went up frequently, a convenient escape from their feverish schedules.

These days, McKusick says the main reason he conducts the tours is to impart to others a sense of the school's history. While he has escorted non-Hopkins-affiliated groups on his tour, he primarily takes students, telling them "it's the pinnacle of the clinical experience at Hopkins."

One fairly recent trip was seen by millions of television viewers. The ABC News television crew that filmed the Hopkins 24/7 series followed McKusick up into the dome with a group of first-year medical students.

Back at the trap door, McKusick takes out the new key.

"OK. Here we go again," he says.

The students wait in silence, with all eyes on the doctor.

Soon they hear a click.

"It works!" McKusick lets out.

"Yeah!" someone bellows. "Now we will appreciate it even more."

As each emerges from the trap door, the smiles grow broad and the eyes open wide to take in the view. It was worth the wait.

From the top of the dome, you can see nearly all of Baltimore--to the east, the outskirts of Baltimore County; west, the downtown skyline; south, the Inner Harbor; and, gazing north, one can make out another familiar Hopkins dome, that of the university's Gilman Hall.

The sky is clear this day, and the wind relatively calm. The students fan out in a circle and perch their hands upon the railing. "Oh, wow," is a common response, as each starts snapping pictures.

"Can I take a picture with you, Dr. McKusick?" asks Steve Osgood.

"Me, too," says Tara Hebert, who braved an illness to make the trip. "I told myself, one day before I leave here, I'm going up to the top of the dome. And here I am. This is so great."

After his stint as a subject, McKusick encourages the students to pose together while he mans their cameras. Add "photographer" to his resume.

From this vantage point, one can see why McKusick and his classmates came here as often as they did. Spectacular view aside, there is perhaps no better place for a medical student to reflect on the experience of being at Johns Hopkins.

For Kuhn, who is a month away from graduation, the event is particularly symbolic. "This is such a nice time to do this," says Kuhn, who fondly recalls her first days at Hopkins. "It's just so neat."

Kuhn leans over the railing and stares down at the tiny people below. Four years ago, she was one of those pedestrians, making her way to her new school. Now her academic journey has ended, and she is on top of the world.

--Greg Rienzi