·         There will be 3 in-person visits at the University of Pennsylvania and 2 phone check-ins.

·         Visit 1 will be the screening Visit (3.5 hours): A member of our research team, like a doctor or specialist, will review your medical history to see if you qualify. Afterwards, researchers will have you complete neuropsychological (memory) tests and questionnaires about your symptoms. This visit will determine if you are a candidate for the study!

·         Visit 2 will be include an MRI scan and your first dose of the study drug (3 hours): You will be randomly assigned to receive either the medication (sildenafil) or a placebo (like flipping a coin). Neither you nor the researchers will initially know which you receive. This visit will involve magnetic resonance imaging or MRI scans of your brain. MRI allows us to take very detailed pictures of your brain so we can look at things like the structure or the function of how your brain is working. For a short part of these MRI scans (about 10 minutes), you will use a special breathing mouthpiece (like a snorkel) that alternates between regular air and air with 5% carbon dioxide. Inhaling this amount of carbon dioxide for one minute can produce sensations like those experienced after holding your breath for approximately 30 seconds, including shortness of breath, dizziness, and discomfort. This helps us see how your brain blood vessels react to gas changes, helping us see how the medication (or placebo) affect blood flow. You will take your first dose of the medication at this visit in the middle of the scan and be sent home with a supply to take for 4 weeks, three times daily.

·         Visit 3 will be the follow-Up visit (1.5 hours): We will repeat the memory tests and ask about side effects again.

·         We will complete 2 quick Phone calls (10 minutes each) between Visit 2 and Visit 3 to see experiencing any side effects.