Challenge B will include an Ensure shake for the participant to drink with deltaG ketone ester, which has generally recognized as safe (GRAS) supplement status .

Subject will drink the shake. Immediate recall memory testing. Subject will be placed in the scanner. Prior to PET acquisition, a low dose CT scan of the head will be obtained for attenuation correction. Patients will be injected with an intravenous bolus of up to 5-10mCi (370 MBq) (+/- 10%) of [11C]AcAc (over 2 minutes). This is the standard dose used in published clinical studies using [11C]AcAc. The PET acquisition will begin immediately after the tracer injection and last for a total of 30 minutes. 2 ml of blood will be sampled from the forearm vein at 3, 6, 8, 12, 20, and 28 minutes post [11C]AcAc tracer injection. This will be used for quantification of radiation estimates throughout the scan-period

The Free and Cued Selective Reminding Test (FCSRT) measures verbal memory through multiple trials of a list learning task. A list of 16 items is visually presented to the participants, who then recall as many items as possible. On subsequent trials, participants are told only those items they omitted on the previous trial. The procedure continues until the participant recalls all items on two subsequent trials. After approximately 30-minute delay, participants are asked to recall as many items as possible. The number of items recalled after the delay is then summed.

Story Recall: This test is a modification of the episodic memory measure from the Wechsler Memory Scale-Revised (WMS-R). In this modified version, free recall of one short story that consists of 44 bits of information will be elicited immediately after it is read aloud to the participant and again after a thirty-minute delay. The total bits of information from the story that are recalled immediately (maximum score = 44) and after the delay interval (maximum score = 44) are recorded. Six comparable versions of this task have been validated in prior studies with older adults.

Benton Visual Retention Test (BVRT): The BVRT is a multiple-choice visual recognition task. Participants view a visual pattern and are then asked to select the target from an array of four patterns, three of which are distractors. It assesses visuospatial working memory. This recognition format has the advantage assessing visual memory while eliminating the influence of visuomotor and manual dexterity difficulties.

The One Card Learning Test involves a visual learning task sensitive to early-stage cognitive impairment. It is one of several computerized assessments developed by CogState that assess executive function and working memory.