Hyperphenylalaninemia (HPA) is a condition caused by tetrahydrobiopterin (BH 4 ) and phenylalanine hydroxylase (PAH) deficiencies. It is essential that differential diagnosis be conducted to distinguish these two causes of HPA, because BH 4 deficiency is a more severe disease involving progressive neurologic deterioration.

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Background This paper presents the protocol of the 4th round of Iranian Multi-center Osteoporosis Study (IMOS), a national survey with the primary objective of estimating the prevalence of osteoporosis and sarcopenia and their risk factors in a representative sample of urban and rural populations. Methods The target population of the survey is all individuals ≥ 50 years in Iran. A multi-stage random sampling method has been used in the study. We stratified the 31 provinces of the country into 5 strata based on the distribution of their potential risk factors for osteoporosis and randomly selected one or two provinces from each stratum. Then, we invited 2530 people aged ≥ 50 years recruited in the 8th National Survey of None Communicable Diseases (NCD) Risk Factors (STEPs-2021) in the selected provinces to participate in IMOS. Body composition measurements including bone mineral density, muscle mass, and fat mass are measured through Dual-energy X-ray Absorptiometry (DXA) method using HOLOGIC (Discovery and Horizon) devices; and Trabecular Bone Score (TBS) is measured on the DXA scans using iNsight software. Anthropometric measurement and physical examinations are made by a trained nurses and other required information are collected through face-to-face interviews made by trained nurses. Laboratory measurements are made in a central lab. The prevalence of osteoporosis and sarcopenia will be estimated after applying sampling design, non-response, and post-stratification weights to the data. Discussion IMOS will provide valuable information on the prevalence and determinants of osteoporosis and sarcopenia at the national level, and the results can be used in evaluating health system interventions and policymaking in the field of musculoskeletal diseases.

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