On the fifth anniversary of my husband's Alzheimer's diagnosis, I watched him take the hand of our home-care aide and call her his wife. It wasn't the first time he'd mistaken Lena for me. It was the first time it happened in front of the people who had come to hear us renew our vows. I had planned the ceremony as a promise to the man I married, not as a test he could pass. But when Peter looked at me and asked why a stranger was wearing his ring, I had to decide whether keeping my promise meant keeping him at home, even when neither of us was safe there. How do you stay married to someone who no longer knows you're his wife?