The doctor begins with an apology because the questions are rudimentary, almost insultingly so. But Robert Warren, fresh off the battlefield in Afghanistan and a surgeon's table, doesn't seem to mind.

Yes, he knows how old he is: 20. He knows his Army rank: specialist. He knows that it's Thursday, that it's June, that the year is 1020. Quickly, he corrects the small stumble: "It's 2010." He knows that his wife is Brittanie, that she's due with their first child any day now, and that they "got married two to three weeks before I went to that country."

Stumble No. 2: "That country."

David Williamson doesn't let it slide. "Which country?"

"Whatever country it was that I got blown up in," Warren says.

In a conference room at the National Naval Medical Center in Bethesda, he purses his lips, and as he searches for the word "Afghanistan," he slides his hand over the left side of his head, which is cratered, like an apple with a bite taken out of it.

"Crap, I can't remember," he says finally.

Warren has trouble remembering a lot of things. Which isn't surprising, considering that several pieces of shrapnel tore through his skull after insurgents outside Kandahar blew up his truck with a rocket-propelled grenade in May. One piece came to rest in the center of Warren's brain - two millimeters from his carotid artery - where it remains, suspended like a piece of fruit in a gelatin mold, too dangerous to extract.

"I'm going to say three words and then have you say them back to me, okay?" says Williamson, a neuropsychiatrist who runs Bethesda's traumatic brain injury unit. "Apple. Desk. Rainbow."

Warren doesn't hesitate: "Apple. Desk. Rainbow."

He seems satisfied to have answered a question correctly. But repeating the words immediately isn't the point of the exercise; it's being able to repeat them in 10 minutes or so, after some other tests. A person with normal cognitive function will probably remember all three words. Patients with mild Alzheimer's might recall two. People with advanced dementia might remember only one, or none at all.

At the Bethesda hospital, the flow of brain-injured patients is constant. For nearly a decade, the United States has been fighting wars in which soldiers are routinely exposed to brain-rattling blasts that can send ripples of compressed air hurtling through the atmosphere at 1,600 feet per second. Now, the military is struggling to come to terms with an often-invisible wound.