Patient description: A 17 year old boy complains of vomiting, non-bloody diarrhea, abdominal pain, fever, chills and loss of appetite for the past 3 days. He ate a salad at a restaurant prior to his diarrhea onset. Physical exam was remarkable for pallor, jaundice, and diffuse abdominal tenderness. Lab results were as follows: Hemoglobin: 9.7 g/dL Platelet: 110,000 /cu.mm Creatinine: 3.6 mg/dL blood urea nitrogen (BUN): 73 mg/dL direct bilirubin: 2.4 mg/dL lactate dehydrogenase (LDH): 881 IU/L (normal: 110-265 IU/L) Peripheral blood smear showed a moderate number of schistocytes and helmet cells. Shiga-like toxin-producing E. coli (STEC) stx1/stx2 were found in stools. He has no other underlying disease and he is not on any medications.

Clinical trial eligibility criteria: eligible ages (years): 0.0-30.0,  Central Nervous System Neoplasm Infantile Fibrosarcoma Recurrent Acute Leukemia Refractory Acute Leukemia Solid Neoplasm A: Patients must have a histologic diagnosis of infantile fibrosarcoma with an NTRK1, NTRK2, or NTRK3 fusion identified in a Clinical Laboratory Improvement Act/College of American Pathologists (CLIA/CAP) certified laboratory. Fusions may be identified by fluorescence in situ hybridization (FISH) or molecular techniques (reverse transcriptase-polymerase chain reaction [RT-PCR] using primers flanking the fusion junction or next generation sequencing). For fusions identified by FISH, an ETV6 rearrangement is sufficient for in Cohort A. Identification of the upstream TRK fusion partner is not required B: Patients must have a histologic diagnosis of any solid tumor other than infantile fibrosarcoma, including central nervous system (CNS) tumors but excluding high grade gliomas. An NTRK1, NTRK2, or NTRK3 fusion must be identified in a CLIA/CAP certified laboratory. Fusions may be identified by FISH or molecular techniques (RT-PCR using primers flanking the fusion junction or next generation sequencing). For fusions identified by FISH, there must be an identified rearrangement in NTRK1, NTRK2, or NTRK3 (e.g., an ETV6 rearrangement is not sufficient for eligibility) unless the patient has a diagnosis of congenital mesoblastic nephroma in which case an ETV6 rearrangement is sufficient for eligibility. Identification of the upstream TRK fusion partner is not required C: Patients must have a histologic diagnosis of relapsed or refractory acute leukemia with an NTRK1, NTRK2, or NTRK3 fusion identified in a CLIA/CAP certified laboratory. Fusions may be identified by FISH or molecular techniques (RT-PCR using primers flanking the fusion junction or next generation sequencing). For fusions identified by FISH, there must be an identified rearrangement in NTRK1, NTRK2, or NTRK3 (e.g., an ETV6 rearrangement is not sufficient for eligibility). Identification of the upstream TRK fusion partner is not required SOLID (COHORTS A AND B): Patients must have measurable disease. Patients must have disease that cannot be completely resected without a predicted functional, neurologic, or significant cosmetic deficit in the opinion of the investigator (COHORT C): Patients must have >= 5% blasts in the bone marrow. Extramedullary disease is permitted Patients must have a Lansky or Karnofsky performance status score of >= 50, corresponding to Eastern Cooperative Oncology Group (ECOG) categories 0, 1 or 2. Use Karnofsky for patients > 16 years of age and Lansky for patients =< 16 years of age. NOTE: Neurologic deficits in patients with CNS tumors must have been stable for at least 7 days prior to study enrollment. Patients who are unable to walk because of paralysis, but who are up in a wheelchair, will be considered ambulatory for the purpose of assessing the performance score A AND B: No prior anti-cancer therapy, including radiotherapy, other than surgical resection is permitted Patients who experience recurrence after surgery alone and no other anti-cancer therapy will be eligible If not eligible due to prior anticancer therapy, patients may be eligible for the larotrectinib arm of Pediatric MATCH (APEC1621A) or treatment with commercial larotrectinib off study C: Patients with relapsed leukemia (Cohort C) must have fully recovered from the acute toxic effects of all prior anti-cancer therapy and must meet the following minimum duration from prior anti-cancer directed therapy prior to enrollment. If after the required timeframe, the numerical are met, e.g. blood count the patient is considered to have recovered adequately  Corticosteroids: If used to modify immune adverse events related to prior therapy, >= 14 days must have elapsed since last dose of corticosteroid. A waiting period prior to enrollment is not required for patients receiving corticosteroid for leukemia therapy/cytoreduction Hematopoietic growth factors: >= 14 days after the last dose of a long-acting growth factor (e.g. pegfilgrastim) or 7 days for short-acting growth factor. For agents that have known adverse events occurring beyond 7 days after administration, this period must be extended beyond the time during which adverse events are known to occur. The duration of this interval must be discussed with the study chair and the study-assigned research coordinator Interleukins, interferons and cytokines (other than hematopoietic growth factors): >= 21 days after the completion of interleukins, interferon or cytokines (other than hematopoietic growth factors ) Stem cell infusions (with or without total body irradiation [TBI]) Allogeneic (non-autologous) bone marrow or stem cell transplant, or any stem cell infusion including donor lymphocyte infusion (DLI) or boost infusion: >= 84 days after infusion and no evidence of graft versus host disease (GVHD) Autologous stem cell infusion including boost infusion: >= 42 days Cellular therapy: >= 42 days after the completion of any type of cellular therapy (e.g., modified T cells, natural killer [NK] cells, dendritic cells, etc.) Radiation therapy (XRT)/external beam irradiation including protons: >= 14 days after local XRT; >= 150 days after TBI, craniospinal XRT or if radiation to >= 50% of the pelvis; >= 42 days if other substantial BM radiation Radiopharmaceutical therapy (e.g., radiolabeled antibody): >= 42 days after systemically administered radiopharmaceutical therapy Patients must not have received prior exposure to TRK inhibitors (including larotrectinib, LOXO-195, entrectinib, lorlatinib, crizotinib, or lestaurtinib)