Can a common diuretic protect the liver and heart in children born with one heart chamber?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

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 1A (formerly part of study drug group who wish continued participation in the observational group): * Subjects who were enrolled in this study in Cohort 2 and are either non-compliant with the medication, no longer want to take the medication, or have an AE that requires them to stop the medication, and patient's family would like to continue participation * Patients were on study medication for 6 weeks or less. * The principal investigator deems it appropriate for the patient to switch to the observational arm. * Patients signing the observational informed consent form. Cohort 1B (observational group - in other studies with intervention): * Subjects between 1 and ≤6 years of age of either gender. * Either single left or single right ventricle. * Subjects who are planned to undergo a Fontan operation at CHOP. * Patients in other interventional studies approved by principal investigator. * Patients signing the observational informed consent form. 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 * Subjects in any study that would preclude participation in the study by altering results Cohort 1A (formerly part of study drug group who wish continued participation in the observational group): * 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). * Subjects in any study that would preclude participation in the current study. Cohort 1B (observational group - in other studies with intervention): * 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 in any study that would preclude participation in the current study or studies not approved by principal investigator. 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). * Subjects in any study that would preclude participation in the study by altering results 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.