Inclusion Criteria:  Single Ventricle (SV) Patients  Cohort 1 (Observational Group - no study medication):  Subjects between 1 and <6 years of age of either gender. Either single left or single right ventricle. Subjects who are scheduled to undergo a Fontan operation at CHOP. Parents signing informed consent.  Cohort 2 (study drug Group - spironolactone):  Subjects between 1 and <6 years of age of either gender. Either single left or single right ventricle. Subjects who are scheduled to undergo a Fontan operation at CHOP. Parents signing informed consent.  Controls  Subjects between 1 and <6 years of age of either gender Subjects with normal ventricular function and normal livers who present to CMR for clinical indications and require anesthesia. Receiving contrast for clinical purposes. No control patient will receive contrast for research purposes. Parents signing informed consent.  Exclusion Criteria:  -  Cohort 1 (Observational Group - no study medication):  Subjects with any condition judged by the patient's physician that would cause this trial to be detrimental to the patient. Any contradiction to a sedated CMR (i.e. presence of a pacemaker). Patient currently taking spironolactone or eplerenone  Cohort 2 (Study Drug Group - Spironolactone):  Subjects with any condition judged by the patient's physician that would cause this trial to be detrimental to the patient. Any contradiction to a sedated CMR (i.e. presence of a pacemaker). Patient currently taking spironolactone or eplerenone Subjects with hyperkalemia or Addison disease; Subjects on enalapril or other angiotensin receptor blockers Subjects with a history of hypersensitivity to spironolactone suspension or any component of the formulation Subjects with a clinically documented diagnosis of severe renal insufficiency (implying estimated glomerular filtration rate (eGFR) <30 mL/minute/1.73 m2).  Controls  Any condition judged by the patient's physician that would cause this trial to be detrimental to the patient. Any contradiction to a sedated CMR (i.e. presence of a pacemaker). Patient currently taking spironolactone, eplerenone or an angiotensin converting the enzyme inhibitor/angiotensin receptor blocker.