Environmental and Nutritional Pathology: Severe Acute Malnutrition and Vitamin D Disorders

Comprehensive practice flashcards covering childhood severe acute malnutrition (marasmus and kwashiorkor), vitamin D metabolism, and the pathophysiology of rickets and osteomalacia.

Marasmus

A form of severe acute malnutrition that develops when the diet is severely lacking in total calories, leading to growth retardation and catabolism of the somatic protein compartment.

Somatic protein compartment

The skeletal muscle protein reservoir that undergoes catabolism and depletion in marasmus as an adaptive response to provide the body with amino acids for energy.

Visceral protein compartment

The internal organ protein pool that is critical for survival; it is depleted only marginally in marasmus, but severely depleted in kwashiorkor.

Serum albumin in marasmus

Circulating protein levels that remain normal or only slightly reduced due to the relative preservation of the visceral protein compartment.

Subcutaneous fat mobilization in marasmus

The breakdown and utilization of subcutaneous adipose stores as an energy fuel, resulting in marked emaciation of the extremities.

Leptin in marasmus

An adipokine whose production is diminished in marasmus, which may stimulate the hypothalamic-pituitary-adrenal axis to release high levels of cortisol.

Cortisol in marasmus

A glucocorticoid produced at elevated levels via hypothalamic-pituitary-adrenal axis stimulation that drives lipolysis and subcutaneous fat loss.

Physical appearance in marasmus

An emaciated habitus with wasted muscle and subcutaneous fat in the extremities, causing the head to appear disproportionately large for the body.

T cell-mediated immune deficiency in marasmus

An impairment of cellular immunity resulting from severe calorie malnutrition that predisposes the patient to concurrent secondary infections.

Kwashiorkor

A form of severe acute malnutrition that occurs when protein deprivation is relatively greater than the reduction in total calories, classically seen in early-weaned infants.

Ga language origin of 'kwashiorkor'

A Ghanese term describing the illness observed in an older child who is displaced from breastfeeding when a new baby is born.

Etiology of kwashiorkor

Severe protein restriction alongside an almost exclusively carbohydrate diet, highly prevalent in impoverished areas of Africa, Southeast Asia, and Central America.

Secondary causes of kwashiorkor

Non-dietary conditions leading to marked protein malabsorption or chronic loss, including chronic diarrheal states, protein-losing enteropathies, the nephrotic syndrome, and extensive burns.

Fad diet-induced kwashiorkor

Rare occurrences of profound protein malnutrition reported in the United States resulting from inappropriate fad diets or replacing milk with rice-based beverages.

Visceral protein compartment in kwashiorkor

The organ protein pool (particularly in the liver) that suffers severe catabolic loss, directly causing pronounced hypoalbuminemia.

Hypoalbuminemia in kwashiorkor

A marked drop in serum albumin resulting from visceral protein loss that reduces plasma oncotic pressure and induces edema.

Masked weight loss in kwashiorkor

The diagnostic pitfall where a patient's true weight loss ($60\%$ to $80\%$ of normal) is obscured by widespread fluid retention and edema.

Sparing of subcutaneous fat and muscle in kwashiorkor

The relative preservation of somatic adipose and skeletal muscle tissue compared with marasmus, though modest losses may also be masked by edema.

Skin lesions of kwashiorkor

Characteristic dermal lesions displaying alternating zones of hyperpigmentation, desquamation, and hypopigmentation (flaky paint dermatosis).

Hair changes in kwashiorkor

A set of changes including loss of pigmentation, alternating light and dark bands, straightening, fine texture, and loss of firm attachment to the scalp.

Fatty liver in kwashiorkor

Hepatomegaly caused by triglyceride accumulation in hepatocytes secondary to defective synthesis of the carrier apoprotein component of lipoproteins.

Neuropsychiatric changes in kwashiorkor

Behavioral symptoms characteristic of kwashiorkor, specifically including apathy, listlessness, and loss of appetite.

Childhood malnutrition phenotypes (FIG. 7.17 and eFIG. 7.3)

Distinct presentations: (A) Marasmus showing emaciation and relative macrocephaly; (B) Kwashiorkor with ascites and generalized edema; and (eFIG. 7.3) characteristic desquamating skin lesions.

Vitamin D

A fat-soluble secosteroid hormone essential for maintaining adequate plasma levels of calcium and phosphorus to support metabolic, neuromuscular, and skeletal functions.

Major functions of vitamin D

The regulation and maintenance of plasma calcium and phosphorus levels to facilitate bone mineralization, neuromuscular transmission, and general cellular metabolism.

Hypocalcemic tetany

A neuromuscular disorder characterized by continuous muscle excitation resulting from a decrease in extracellular ionized calcium when vitamin D is deficient.

Ionized calcium in tetany

The free extracellular calcium pool whose reduction lowers muscle excitation thresholds, triggering tetanic spasms.

Compensatory PTH response to hypocalcemia

The increased secretion of parathyroid hormone (PTH) that stimulates osteoclastic bone resorption to normalize serum calcium, making bone changes dominate and tetany rare.

Endogenous synthesis of vitamin D

The cutaneous photochemical pathway that provides approximately $90\%$ of the human requirement under usual conditions of sun exposure.

7-Dehydrocholesterol

The epidermal precursor compound that undergoes photochemical conversion via solar or artificial ultraviolet (UV) radiation into cholecalciferol (vitamin $D_3$).

Cholecalciferol (Vitamin $D_3$)

The form of vitamin D generated endogenously in the skin from 7-dehydrocholesterol following ultraviolet light exposure.

Melanin effect on vitamin D synthesis

The competitive absorption of ultraviolet light by cutaneous melanin in dark-skinned individuals, reducing endogenous vitamin D production.

Dietary sources of vitamin D

Exogenous nutritional sources supplying a small fraction of the daily requirement, including deep-sea fish, plants, grains, and fortified milk.

Ergosterol

A provitamin sterol present in plant sources that is ingested and converted into active vitamin D within the human body.

Vitamin D-binding protein (DBP)

A circulating plasma $\alpha_1$-globulin that binds vitamin D and its hydroxylated metabolites to transport them to the liver and kidneys.

Hepatic 25-hydroxylase (25-OHase)

The hepatic enzyme responsible for converting cholecalciferol into 25-hydroxyvitamin D [$25\text{-(OH)-D}$].

25-Hydroxyvitamin D [$25\text{-(OH)-D}$]

The predominant circulating and storage metabolite of vitamin D produced in the liver prior to kidney activation.

Renal $\alpha_1$-hydroxylase (1-OHase)

The proximal renal tubular enzyme that converts $25\text{-(OH)-D}$ into $1,25\text{-dihydroxyvitamin D}$, the most biologically active form.

1,25-Dihydroxyvitamin D [$1,25\text{-(OH)}_2\text{-D}$]

The active secosteroid hormone that binds nuclear receptors in target tissues to regulate calcium and phosphorus balance.

Hypocalcemia-mediated activation of $\alpha_1$-hydroxylase

An endocrine reflex where decreased serum calcium stimulates PTH release, which directly activates renal $\alpha_1$-hydroxylase.

Hypophosphatemia-mediated activation of $\alpha_1$-hydroxylase

A direct feedback pathway where decreased plasma phosphorus activates renal $\alpha_1$-hydroxylase independently of PTH.

Feedback downregulation of $1,25\text{-(OH)}_2\text{-D}$

An autoregulatory feedback loop wherein elevated levels of active $1,25\text{-(OH)}_2\text{-D}$ inhibit renal $\alpha_1$-hydroxylase activity.

Nuclear vitamin D receptor (VDR) mechanism

High-affinity intracellular receptors in nucleated cells that bind $1,25\text{-(OH)}_2\text{-D}$ and attach to target DNA regulatory elements to induce gene transcription.

Intestinal calcium absorption by $1,25\text{-(OH)}_2\text{-D}$

The transcriptional upregulation of enterocyte calcium transport proteins, enhancing intestinal uptake of calcium and phosphorus.

Calbindin

An intracellular calcium-binding chaperone protein upregulated by $1,25\text{-(OH)}_2\text{-D}$ that facilitates calcium movement across enterocytes and renal tubular cells.

Renal distal tubular calcium resorption

The active reabsorption of calcium promoted by $1,25\text{-(OH)}_2\text{-D}$ via epithelial calcium channels and plasma membrane calcium pumps.

PTH gene regulation by $1,25\text{-(OH)}_2\text{-D}$

A negative feedback action on parathyroid chief cells in which increased $1,25\text{-(OH)}_2\text{-D}$ decreases transcription of the PTH gene.

RANK/RANKL pathway regulation by vitamin D

The upregulation of RANK ligand (RANKL) on osteoblasts by $1,25\text{-(OH)}_2\text{-D}$, which binds RANK on osteoclast precursors to induce osteoclastogenesis and bone resorption.

Dual skeletal action of vitamin D

The ability of vitamin D to stimulate bone resorption during hypocalcemia to preserve plasma levels, while driving bone matrix mineralization during normocalcemia.

Secondary hyperparathyroidism in vitamin D deficiency

Compensatory overproduction of PTH provoked by low calcium that restores blood calcium via bone breakdown but induces severe urinary phosphate excretion.

Urinary phosphate wasting

PTH-induced inhibition of renal phosphate reabsorption, causing persistent hypophosphatemia and inadequate hydroxyapatite formation.

Sunlight deprivation in vitamin D deficiency

The leading worldwide cause of rickets and osteomalacia, prevalent in high latitudes, deeply covered dress customs, or strict sunscreen usage.

Maternal risk factors for infant rickets

Frequent pregnancies followed by prolonged lactation, which depletes maternal vitamin D stores and results in breast milk deficient in vitamin D.

Non-nutritional etiologies of rickets and osteomalacia

Systemic diseases including chronic renal failure (defective $\alpha_1$-hydroxylase activity), urinary phosphate wasting disorders, and gastrointestinal malabsorption.

Vitamin D receptor (VDR) genetic polymorphisms

Inherited receptor variants linked to accelerated bone mineral loss with aging and susceptibility to familial forms of osteoporosis.

Intramembranous ossification

The developmental mechanism of flat bone formation where mesenchymal cells differentiate into osteoblasts to synthesize unmineralized osteoid directly.

Endochondral ossification

The process forming long bones wherein cartilaginous growth plates undergo provisional calcification followed by osteoid replacement and mineralization.

Fundamental derangement in rickets and osteomalacia

An abnormal accumulation of excess unmineralized bone matrix (osteoid) caused by insufficient calcium-phosphate deposition.

Rickets

A childhood bone disorder occurring prior to epiphyseal fusion, defined by defective provisional calcification of growth plate cartilage and skeletal deformities.

Cartilage palisade disruption in rickets

The loss of orderly chondrocyte columnar stacking at the costochondral junction, resulting in disorganized clusters of cartilage cells.

Epiphyseal cartilage overgrowth in rickets

The excessive accumulation of hypertrophic chondrocytes caused by failure of provisional calcification and delayed cellular apoptosis.

Rachitic rosary

A bead-like deformity of the chest wall produced by cartilage overgrowth and uncalcified osteoid expansion at the costochondral junctions.

Anterior sternal protrusion in rickets

Pigeon breast deformity caused by the inward traction of respiratory muscles on softened, pliable rachitic ribs that pushes the sternum anteriorly.

Harrison groove

A transverse skeletal depression girdling the lower thoracic cage, caused by the inward pull of the diaphragmatic attachment on softened rachitic ribs.

Craniotabes

Thinning and softening of the cranial bones (occipital and parietal) in infantile rickets that yield to finger pressure and snap back via elastic recoil.

Frontal bossing

A squared appearance of the rachitic child's calvarium caused by an excess deposition of unmineralized osteoid along the frontal bones.

Ambulatory deformities in rickets

Skeletal anomalies that appear when weight-bearing begins, notably anterior pelvic tilting, exaggerated lumbar lordosis, and outward bowing of the legs.

Osteomalacia

A metabolic bone disease of adults characterized by inadequate mineralization of newly formed osteoid laid down during ongoing skeletal remodeling.

Fracture vulnerability in osteomalacia

The marked susceptibility of structurally weak osteomalacic bone to microfractures and gross fractures, most commonly involving the vertebral bodies and femoral necks.

Histologic appearance of osteomalacia

A thickened seam of eosinophilic unmineralized osteoid matrix surrounding more basophilic, normally mineralized trabecular bone cores.