Open Label Trial Assessing Safety and Efficacy of Burosumab (KRN23), in a Patient With ENS and Hypophosphatemic Rickets

Inclusion Criteria: 1. Patient has confirmed ENS by physician diagnosis 2. Patient has confirmed FGF23 elevations in the context of low serum phosphorous \< 4.1 mg/dL 3. Patient able to tolerate KRN23 treatment 4. Have a corrected serum calcium level \< 10.8mg/dL 5. Have an eGFR \>60 ml/min 6. Must be willing in the opinion of the investigator, to comply with study procedures and schedule 7. Provide written informed consent by a parent after the study has been explained and prior to any research related procedures begin Exclusion Criteria: 1. Concomitant use of active vitamin D (i.e calcitriol) and/or exogenous phosphate supplementation. Patient will be allowed OTC Vitamin D should levels drop below \<20 ng/ml 2. The use or enrollment in studies using other investigational therapies including other monoclonal antibodies 3. Subject and their parent not willing or not able to give written informed consent 4. In the Investigators opinion, the patient may not be able to meet all the requirements for study participation 5. Patient has a history of hypersensitivity to KRN23 excipients that in the opinion of the investigator, places the patient at an increased risk of adverse effects 6. Patient has a condition that in the opinion of the investigator could present a concern for subject safety or data interpretation.