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Deep convection in the Asian summer monsoon is a significant transport process for lifting pollutants from the planetary boundary layer to the tropopause level. This process enables efficient injection into the stratosphere of reactive species such as chlorinated very-short-lived substances (Cl-VSLSs) that deplete ozone. Past studies of convective transport associated with the Asian summer monsoon have focused mostly on the south Asian summer monsoon. Airborne observations reported in this work identify the East Asian summer monsoon convection as an effective transport pathway that carried record-breaking levels of ozone-depleting Cl-VSLSs (mean organic chlorine from these VSLSs ~500 ppt) to the base of the stratosphere. These unique observations show total organic chlorine from VSLSs in the lower stratosphere over the Asian monsoon tropopause to be more than twice that previously reported over the tropical tropopause. Considering the recently observed increase in Cl-VSLS emissions and the ongoing strengthening of the East Asian summer monsoon under global warming, our results highlight that a reevaluation of the contribution of Cl-VSLS injection via the Asian monsoon to the total stratospheric chlorine budget is warranted.
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PURPOSE: To review a multicentric series of lateral-type posterior fossa ependymomas operated in the last ten years and to analyze the factors related to clinical evolution and tumor survival. METHODS: Descriptive, retrospective study. Active members of the Spanish Society of Pediatric Neurosurgery were invited to participate in this multicentric study. Clinical and radiological data were incorporated to an open database. The role of histologic grade, grade of resection, postoperative morbidities, and clinical follow-up was evaluated through bivariate associations (chi-square), Kaplan-Meier's curves (log-rank test), and multivariate analysis (binary logistic regression). RESULTS: Fourteen centers entered the study, and 25 cases with a minimum follow-up of 6 months were included. There were 13 boys and 12 girls with a mean age close to 3 years. Mean tumor volume at diagnosis was over 60 cc. A complete resection was achieved in 8 patients and a near-total resection in 5 cases. Fifteen tumors were diagnosed as ependymoma grade 2 and ten as ependymoma grade 3. Major morbidity occurred postoperatively in 14 patients but was resolved in twelve within 6 months. There were six cases of death and 11 cases of tumor progression along the observation period. Mean follow-up was 44.8 months. Major morbidity was significantly associated with histologic grade but not with the degree of resection. Overall and progression-free survival were significantly associated with complete surgical resection. At the last follow-up, 16 patients carried a normal life, and three displayed a mild restriction according to Lansky's scale. CONCLUSIONS: Lateral-type posterior fossa ependymomas constitute a specific pathologic and clinical tumor subtype with bad prognosis. Gross total resection is the goal of surgical treatment, for it significantly improves prognosis with no additional morbidity. Neurological deficits associated to lower cranial nerve dysfunction are common, but most are transient. Deeper genetic characterization of these tumors may identify risk factors that guide new treatments and stratification of adjuvant therapies.
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Ependimoma , Masculino , Femenino , Humanos , Niño , Estudios Retrospectivos , Pronóstico , Terapia Combinada , Supervivencia sin Progresión , Ependimoma/cirugía , Ependimoma/patologíaRESUMEN
Decades of air quality improvements have substantially reduced the motor vehicle emissions of volatile organic compounds (VOCs). Today, volatile chemical products (VCPs) are responsible for half of the petrochemical VOCs emitted in major urban areas. We show that VCP emissions are ubiquitous in US and European cities and scale with population density. We report significant VCP emissions for New York City (NYC), including a monoterpene flux of 14.7 to 24.4 kg â d-1 â km-2 from fragranced VCPs and other anthropogenic sources, which is comparable to that of a summertime forest. Photochemical modeling of an extreme heat event, with ozone well in excess of US standards, illustrates the significant impact of VCPs on air quality. In the most populated regions of NYC, ozone was sensitive to anthropogenic VOCs (AVOCs), even in the presence of biogenic sources. Within this VOC-sensitive regime, AVOCs contributed upwards of â¼20 ppb to maximum 8-h average ozone. VCPs accounted for more than 50% of this total AVOC contribution. Emissions from fragranced VCPs, including personal care and cleaning products, account for at least 50% of the ozone attributed to VCPs. We show that model simulations of ozone depend foremost on the magnitude of VCP emissions and that the addition of oxygenated VCP chemistry impacts simulations of key atmospheric oxidation products. NYC is a case study for developed megacities, and the impacts of VCPs on local ozone are likely similar for other major urban regions across North America or Europe.
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Contaminantes Atmosféricos/análisis , Ozono , Compuestos Orgánicos Volátiles/análisis , Contaminantes Atmosféricos/química , Contaminación del Aire , Ciudades , Monitoreo del Ambiente/métodos , Europa (Continente) , Humanos , Modelos Teóricos , Monoterpenos/análisis , Ciudad de Nueva York , Óxidos de Nitrógeno/análisis , Óxidos de Nitrógeno/química , Odorantes/análisis , Densidad de Población , Emisiones de Vehículos/análisis , Compuestos Orgánicos Volátiles/químicaRESUMEN
CAD is a large, 2225 amino acid multienzymatic protein required for de novo pyrimidine biosynthesis. Pathological CAD variants cause a developmental and epileptic encephalopathy which is highly responsive to uridine supplements. CAD deficiency is difficult to diagnose because symptoms are nonspecific, there is no biomarker, and the protein has over 1000 known variants. To improve diagnosis, we assessed the pathogenicity of 20 unreported missense CAD variants using a growth complementation assay that identified 11 pathogenic variants in seven affected individuals; they would benefit from uridine treatment. We also tested nine variants previously reported as pathogenic and confirmed the damaging effect of seven. However, we reclassified two variants as likely benign based on our assay, which is consistent with their long-term follow-up with uridine. We found that several computational methods are unreliable predictors of pathogenic CAD variants, so we extended the functional assay results by studying the impact of pathogenic variants at the protein level. We focused on CAD's dihydroorotase (DHO) domain because it accumulates the largest density of damaging missense changes. The atomic-resolution structures of eight DHO pathogenic variants, combined with functional and molecular dynamics analyses, provided a comprehensive structural and functional understanding of the activity, stability, and oligomerization of CAD's DHO domain. Combining our functional and protein structural analysis can help refine clinical diagnostic workflow for CAD variants in the genomics era.
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Dihidroorotasa , Proteínas , Humanos , Dihidroorotasa/química , Dihidroorotasa/genética , Dihidroorotasa/metabolismo , Mutación Missense , UridinaRESUMEN
The evolution of organic aerosol (OA) and brown carbon (BrC) in wildfire plumes, including the relative contributions of primary versus secondary sources, has been uncertain in part because of limited knowledge of the precursor emissions and the chemical environment of smoke plumes. We made airborne measurements of a suite of reactive trace gases, particle composition, and optical properties in fresh western US wildfire smoke in July through August 2018. We use these observations to quantify primary versus secondary sources of biomass-burning OA (BBPOA versus BBSOA) and BrC in wildfire plumes. When a daytime wildfire plume dilutes by a factor of 5 to 10, we estimate that up to one-third of the primary OA has evaporated and subsequently reacted to form BBSOA with near unit yield. The reactions of measured BBSOA precursors contribute only 13 ± 3% of the total BBSOA source, with evaporated BBPOA comprising the rest. We find that oxidation of phenolic compounds contributes the majority of BBSOA from emitted vapors. The corresponding particulate nitrophenolic compounds are estimated to explain 29 ± 15% of average BrC light absorption at 405 nm (BrC Abs405) measured in the first few hours of plume evolution, despite accounting for just 4 ± 2% of average OA mass. These measurements provide quantitative constraints on the role of dilution-driven evaporation of OA and subsequent radical-driven oxidation on the fate of biomass-burning OA and BrC in daytime wildfire plumes and point to the need to understand how processing of nighttime emissions differs.
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Contaminantes Atmosféricos/química , Carbono/análisis , Humo , Incendios Forestales , Aerosoles , Monitoreo del Ambiente , Material Particulado , Estados UnidosRESUMEN
Nitrogen oxides are essential for the formation of secondary atmospheric aerosols and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere. Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides. However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than are observed. A 'renoxification' process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. Here we present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calculations based on the Master Chemical Mechanism suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions. Given that oceans account for more than 70 per cent of Earth's surface, we propose that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Recycling of nitrogen oxides in remote oceanic regions with minimal direct nitrogen oxide emissions could increase the formation of tropospheric oxidants and secondary atmospheric aerosols on a global scale.
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Atmósfera/química , Óxidos de Nitrógeno/análisis , Óxidos de Nitrógeno/química , Nitrógeno/análisis , Nitrógeno/química , Agua de Mar/química , Aerosoles/química , Océano Atlántico , Nitratos/análisis , Nitratos/química , Ácido Nítrico/química , Ácido Nitroso/análisis , Ácido Nitroso/química , North Carolina , Oxidantes/química , Fotólisis , South CarolinaRESUMEN
Sulfate ([Formula: see text]) and nitrate ([Formula: see text]) account for half of the fine particulate matter mass over the eastern United States. Their wintertime concentrations have changed little in the past decade despite considerable precursor emissions reductions. The reasons for this have remained unclear because detailed observations to constrain the wintertime gas-particle chemical system have been lacking. We use extensive airborne observations over the eastern United States from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign; ground-based observations; and the GEOS-Chem chemical transport model to determine the controls on winter [Formula: see text] and [Formula: see text] GEOS-Chem reproduces observed [Formula: see text]-[Formula: see text]-[Formula: see text] particulate concentrations (2.45 µg [Formula: see text]) and composition ([Formula: see text]: 47%; [Formula: see text]: 32%; [Formula: see text]: 21%) during WINTER. Only 18% of [Formula: see text] emissions were regionally oxidized to [Formula: see text] during WINTER, limited by low [H2O2] and [OH]. Relatively acidic fine particulates (pHâ¼1.3) allow 45% of nitrate to partition to the particle phase. Using GEOS-Chem, we examine the impact of the 58% decrease in winter [Formula: see text] emissions from 2007 to 2015 and find that the H2O2 limitation on [Formula: see text] oxidation weakened, which increased the fraction of [Formula: see text] emissions oxidizing to [Formula: see text] Simultaneously, NOx emissions decreased by 35%, but the modeled [Formula: see text] particle fraction increased as fine particle acidity decreased. These feedbacks resulted in a 40% decrease of modeled [[Formula: see text]] and no change in [[Formula: see text]], as observed. Wintertime [[Formula: see text]] and [[Formula: see text]] are expected to change slowly between 2015 and 2023, unless [Formula: see text] and NOx emissions decrease faster in the future than in the recent past.
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Wildfires have a significant adverse impact on air quality in the United States (US). To understand the potential health impacts of wildfire smoke, many epidemiology studies rely on concentrations of fine particulate matter (PM) as a smoke tracer. However, there are many gas-phase hazardous air pollutants (HAPs) identified by the Environmental Protection Agency (EPA) that are also present in wildfire smoke plumes. Using observations from the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN), a 2018 aircraft-based field campaign that measured HAPs and PM in western US wildfire smoke plumes, we identify the relationships between HAPs and associated health risks, PM, and smoke age. We find the ratios between acute, chronic noncancer, and chronic cancer HAPs health risk and PM in smoke decrease as a function of smoke age by up to 72% from fresh (<1 day of aging) to old (>3 days of aging) smoke. We show that acrolein, formaldehyde, benzene, and hydrogen cyanide are the dominant contributors to gas-phase HAPs risk in smoke plumes. Finally, we use ratios of HAPs to PM along with annual average smoke-specific PM to estimate current and potential future smoke HAPs risks.
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Contaminantes Atmosféricos , Contaminación del Aire , Incendios Forestales , Contaminantes Atmosféricos/análisis , Material Particulado/análisis , Humo/efectos adversos , Humo/análisis , Estados UnidosRESUMEN
Wildfires are an important source of nitrous acid (HONO), a photolabile radical precursor, yet in situ measurements and quantification of primary HONO emissions from open wildfires have been scarce. We present airborne observations of HONO within wildfire plumes sampled during the Western Wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN) campaign. ΔHONO/ΔCO close to the fire locations ranged from 0.7 to 17 pptv ppbv-1 using a maximum enhancement method, with the median similar to previous observations of temperate forest fire plumes. Measured HONO to NOx enhancement ratios were generally factors of 2, or higher, at early plume ages than previous studies. Enhancement ratios scale with modified combustion efficiency and certain nitrogenous trace gases, which may be useful to estimate HONO release when HONO observations are lacking or plumes have photochemical exposures exceeding an hour as emitted HONO is rapidly photolyzed. We find that HONO photolysis is the dominant contributor to hydrogen oxide radicals (HOx = OH + HO2) in early stage (<3 h) wildfire plume evolution. These results highlight the role of HONO as a major component of reactive nitrogen emissions from wildfires and the main driver of initial photochemical oxidation.
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Contaminantes Atmosféricos/análisis , Incendios Forestales , Aerosoles , Ácido Nitroso/análisis , HumoRESUMEN
Lysosomal storage diseases (LSDs) are a heterogeneous group of genetic disorders with variable degrees of severity and a broad phenotypic spectrum, which may overlap with a number of other conditions. While individually rare, as a group LSDs affect a significant number of patients, placing an important burden on affected individuals and their families but also on national health care systems worldwide. Here, we present our results on the use of an in-house customized next-generation sequencing (NGS) panel of genes related to lysosome function as a first-line molecular test for the diagnosis of LSDs. Ultimately, our goal is to provide a fast and effective tool to screen for virtually all LSDs in a single run, thus contributing to decrease the diagnostic odyssey, accelerating the time to diagnosis. Our study enrolled a group of 23 patients with variable degrees of clinical and/or biochemical suspicion of LSD. Briefly, NGS analysis data workflow, followed by segregation analysis allowed the characterization of approximately 41% of the analyzed patients and the identification of 10 different pathogenic variants, underlying nine LSDs. Importantly, four of those variants were novel, and, when applicable, their effect over protein structure was evaluated through in silico analysis. One of the novel pathogenic variants was identified in the GM2A gene, which is associated with an ultra-rare (or misdiagnosed) LSD, the AB variant of GM2 Gangliosidosis. Overall, this case series highlights not only the major advantages of NGS-based diagnostic approaches but also, to some extent, its limitations ultimately promoting a reflection on the role of targeted panels as a primary tool for the prompt characterization of LSD patients.
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Marcadores Genéticos , Predisposición Genética a la Enfermedad , Pruebas Genéticas , Variación Genética , Secuenciación de Nucleótidos de Alto Rendimiento/métodos , Enfermedades por Almacenamiento Lisosomal/diagnóstico , Lisosomas/patología , Salud Global , Humanos , Enfermedades por Almacenamiento Lisosomal/genética , Lisosomas/genética , Análisis de Secuencia de ADNRESUMEN
Halogens in the troposphere are increasingly recognized as playing an important role for atmospheric chemistry, and possibly climate. Bromine and iodine react catalytically to destroy ozone (O3), oxidize mercury, and modify oxidative capacity that is relevant for the lifetime of greenhouse gases. Most of the tropospheric O3 and methane (CH4) loss occurs at tropical latitudes. Here we report simultaneous measurements of vertical profiles of bromine oxide (BrO) and iodine oxide (IO) in the tropical and subtropical free troposphere (10 °N to 40 °S), and show that these halogens are responsible for 34% of the column-integrated loss of tropospheric O3. The observed BrO concentrations increase strongly with altitude (â¼ 3.4 pptv at 13.5 km), and are 2-4 times higher than predicted in the tropical free troposphere. BrO resembles model predictions more closely in stratospheric air. The largest model low bias is observed in the lower tropical transition layer (TTL) over the tropical eastern Pacific Ocean, and may reflect a missing inorganic bromine source supplying an additional 2.5-6.4 pptv total inorganic bromine (Bry), or model overestimated Bry wet scavenging. Our results highlight the importance of heterogeneous chemistry on ice clouds, and imply an additional Bry source from the debromination of sea salt residue in the lower TTL. The observed levels of bromine oxidize mercury up to 3.5 times faster than models predict, possibly increasing mercury deposition to the ocean. The halogen-catalyzed loss of tropospheric O3 needs to be considered when estimating past and future ozone radiative effects.
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Encefalopatías , Síndromes de Neurotoxicidad , Adolescente , Anticonvulsivantes , Encefalopatías/diagnóstico , Femenino , HumanosRESUMEN
We use measurements made onboard the National Science Foundation's C-130 research aircraft during the 2013 Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) experiment to examine total Hg (THg) emission ratios (EmRs) for six coal-fired power plants (CFPPs) in the southeastern U.S. We compare observed enhancement ratios (ERs) with EmRs calculated using Hg emissions data from two inventories: the National Emissions Inventory (NEI) and the Toxics Release Inventory (TRI). For four CFPPs, our measured ERs are strongly correlated with EmRs based on the 2011 NEI (r(2) = 0.97), although the inventory data exhibit a -39% low bias. Our measurements agree best (to within ±32%) with the NEI Hg data when the latter were derived from on-site emissions measurements. Conversely, the NEI underestimates by approximately 1 order of magnitude the ERs we measured for one previously untested CFPP. Measured ERs are uncorrelated with values based on the 2013 TRI, which also tends to be biased low. Our results suggest that the Hg inventories can be improved by targeting CFPPs for which the NEI- and TRI-based EmRs have significant disagreements. We recommend that future versions of the Hg inventories should provide greater traceability and uncertainty estimates.
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Aerosoles/análisis , Carbón Mineral/análisis , Mercurio/análisis , Nitrógeno/análisis , Oxidantes/química , Centrales Eléctricas , Contaminantes Atmosféricos/análisis , Atmósfera/química , Dióxido de Carbono/análisis , Modelos Lineales , Sudeste de Estados Unidos , Dióxido de Azufre/análisisRESUMEN
The protective and preservative role of apple skin in maintaining the integrity of the fruit is well-known, with its mechanical behaviour playing a pivotal role in determining fruit storage capacity. This study employs a combination of experimental and numerical methodologies, specifically utilising the digital image correlation (DIC) technique. A specially devised inverse strategy is applied to evaluate the mechanical behaviour of apple skin under uniaxial tensile loading. Three apple cultivars were tested in this work: Malus domestica Starking Delicious, Malus pumila Rennet, and Malus domestica Golden Delicious. Stress-strain curves were reconstructed, revealing distinct variations in the mechanical responses among these cultivars. Yeoh's hyperelastic model was fitted to the experimental data to identify the coefficients capable of reproducing the non-linear deformation. The results suggest that apple skin varies significantly in composition and structure among the tested cultivars, as evidenced by differences in elastic properties and non-linear behaviour. These differences can significantly affect how fruit is handled, stored, and transported. Thus, the insights resulting from this research enable the development of mathematical models based on the mechanical behaviour of apple tissue, constituting important data for improvements in the economics of the agri-food industry.
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In this study we present a proof of concept of a simple and straightforward approach for the development of a Bacterial Nanocellulose drug delivery system (BNC-DDS), envisioning the local delivery of immunomodulatory drugs to prevent foreign body reaction (FBR). Inspired by the self-adhesion behavior of BNC upon drying, we proposed a BNC laminate entrapping commercial crystalline drugs (dexamethasone-DEX and GW2580) in a sandwich system. The stability of the bilayer BNC-DDS was evidenced by the high interfacial energy of the bilayer films, 150 ± 11 and 88 ± 7 J/m2 respectively for 2 mm- and 10-mm thick films, corresponding to an increase of 7.5 and 4.4-fold comparatively to commercial tissue adhesives. In vitro release experiments unveiled the tunability of the bilayer BNC-DDS by showing extended drug release when thicker BNC membranes were used (from 16 to 47 days and from 35 to 132 days, for the bilayer-BNC entrapping DEX and GW2580, respectively). Mathematical modeling of the release data pointed to a diffusion-driven mechanism with non-fickian behavior. Overall, the results have demonstrated the potential of this simple approach for developing BNC-drug depots for localized and sustained release of therapeutic agents over adjustable timeframes.
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BACKGROUND: Breast cancer survivors' (BCS) attitudes toward eMental Health (eMH) are largely unknown, and adoption predictors and their interrelationships remain unclear. This study aimed to explore BCS' attitudes toward eMH and investigate associated variables. METHODS: A cross-sectional study involving 336 Portuguese BCS was conducted. Attitudes toward eMH, depression and anxiety symptoms, health-related quality of life, and sociodemographic, clinical, and internet-related variables were assessed using validated questionnaires. Spearman-ranked correlations, χ2, and multiple regression analyses were computed to explore associations between attitudes and collected variables. RESULTS: BCS held a neutral stance toward eMH. In models adjusted for age and education, positive attitudes were statistically significantly associated with increased depressive symptoms and worse emotional, cognitive, and body image functioning. Social network use, online health information and mental healthcare seeking, higher self-reported knowledge of eMH, and previous use of remote healthcare were positively associated with better attitudes toward eMH. CONCLUSIONS: eMH programs targeting BCS seem to be a promising strategy for providing supportive psychosocial care to BCS. However, increasing awareness about eMH efficacy and security may be necessary to improve its acceptance and use among BCS. Additional research is necessary to understand how BCS' unmet care needs, and specifically their psychological distress severity, may impact BCS' acceptance and use of eMH.
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Glucose homeostasis is essential for energy production and the central nervous system function, depending on glycogen metabolism. Glycogen storage diseases (GSD) are caused by enzymatic defects of the glycogen degradation and mainly involve the liver since the inhibition of hepatic glycogen breakdown results in its excessive storage and hepatomegaly. Other findings are hypoglycemia and hyperlactatemia and consequent neurological symptoms. GSD Type Ia is a severe disease with clinical manifestations usually occurring in the first months. Morbidity and mortality are high, when not treated. The patient was a male newborn, with nonconsanguineous couple, born by eutocic delivery and weight 3760 g. On Day 2, weight loss >10% and jaundice were noticed, and physical examination was as normal. The investigation showed low glucose that only respond to iv glucose, metabolic acidosis, hyperlactatemia and elevated liver enzymes. Considering his inherited metabolic disease, he was transferred to the Reference Center. Complementary tests showed hypertriglyceridemia and absence of ketone bodies. Abdominal US revealed a liver in the upper limit of normal. Most likely clinical diagnosis was GSD type Ia, confirmed by genetic test. He needed iv glucose, but then stabilized with formula without galactose, supplemented with dextrin every 2 hours. He is now 7 months old, has flash glucose self-monitoring system, maintaining frequent feedings, with sporadic hypoglycemia with normal physical development and no hepatomegaly. Hypoglycemia and early weight loss in newborns are red flags for metabolic diseases or other conditions. When accompanied by other metabolic findings, such as hyperlactatemia and metabolic acidosis, associated with short fasting periods, glycogen metabolism disorders must be considered. Patients with GSD Type Ia generally appear normal at birth and an early presentation is not frequent within the first hours after birth. Moreover, avoiding fasting and hypoglycemia are of vital importance for better cognitive outcome, global prognosis, and prevention of other metabolic abnormalities.
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External cycling regenerating nitrogen oxides (NOx ≡ NO + NO2) from their oxidative reservoir, NOz, is proposed to reshape the temporal-spatial distribution of NOx and consequently hydroxyl radical (OH), the most important oxidant in the atmosphere. Here we verify the in situ external cycling of NOx in various environments with nitrous acid (HONO) as an intermediate based on synthesized field evidence collected onboard aircraft platform at daytime. External cycling helps to reconcile stubborn underestimation on observed ratios of HONO/NO2 and NO2/NOz by current chemical model schemes and rationalize atypical diurnal concentration profiles of HONO and NO2 lacking noontime valleys specially observed in low-NOx atmospheres. Perturbation on the budget of HONO and NOx by external cycling is also found to increase as NOx concentration decreases. Consequently, model underestimation of OH observations by up to 41% in low NOx atmospheres is attributed to the omission of external cycling in models.