Pediatrics System
Pediatrics
High-yield pediatrics for USMLE Step 2/3 — neonatal physiology, breastfeeding, genetic inheritance patterns, X-linked dominant disorders, pediatric infections, child abuse, and pediatric emergencies, extracted from Divine Intervention and organized by clinical cluster.
Neonatal & Developmental
Neonatal jaundice and breastfeeding questions are among the most frequently tested pediatric topics on Step 2/3 — and among the most commonly missed. The key is distinguishing breastfeeding jaundice (first week, inadequate intake, indirect bilirubin) from breast milk jaundice (second week onwards, mild, benign). Phototherapy works for indirect bilirubin only; direct hyperbilirubinemia is always pathological.
EP317
Breastfeeding, Newborn Jaundice & the NBMEs
- Exclusive breastfeeding: Recommended for at least 6 months on NBME exams. Breast milk contains IgA (colostrum = highest IgA). Benefits: ↓ SIDS, ↓ allergies (eczema, asthma), ↓ infections (meningitis, otitis media, NEC, botulism), ↓ celiac disease, ↓ autoimmune disease. Mom benefits: weight loss, ↓ ovarian cancer (↓ ovulations via prolactin → ↓ HPG axis), ↓ breast cancer (↓ estrogen/progesterone)
- Breastfeeding jaundice (first week): F = First week. Inadequate intake → dehydration → ↑ GI transit time → ↑ enterohepatic recirculation of bilirubin → ↑ indirect bilirubin. Cause: not latching well, feeding every 4–6h instead of 2h. Treatment: ensure adequate feeding frequency and volume
- Breast milk jaundice (8–14 days): Child gaining weight, physically normal, mild indirect hyperbilirubinemia. Cause: substance in breast milk inhibits UDPGT. Rarely tested; usually NOT the correct answer on NBMEs
- Physiologic vs pathologic jaundice: Pathologic = ANY direct hyperbilirubinemia, OR jaundice within first 24 hours of life. Most common causes of pathologic jaundice: biliary atresia (most common), choledochal cyst. Biliary atresia → Kasai procedure within first 2–4 weeks or liver transplant
- Phototherapy: Converts trans-bilirubin (water insoluble) → cis-bilirubin (water soluble). Cover eyes during therapy. Indication for exchange transfusion: phototherapy failing + total bilirubin >25. Phototherapy only for INDIRECT bilirubin — never for direct
Neonatal Jaundice — Differential by Timing
| Timing | Type | Cause | Action |
|---|---|---|---|
| <24 hours | Always pathologic | Hemolytic disease (ABO/Rh incompatibility, G6PD, spherocytosis) | Urgent workup |
| Days 2–7 | Physiologic (indirect) or pathologic | Physiologic: normal RBC breakdown; Breastfeeding jaundice: inadequate intake | Phototherapy if severe; increase feeding |
| Days 8–14+ | Usually benign | Breast milk jaundice (inhibits UDPGT); Biliary atresia if direct | Reassure if indirect + healthy; workup if direct |
| Any age | Always pathologic if DIRECT | Biliary atresia, choledochal cyst, Alagille syndrome, neonatal hepatitis | Kasai procedure (biliary atresia within weeks) |
Why Neonates Are Jaundice-Prone
In utero = relative hypoxia (lungs not used) → kidneys produce EPO → high Hgb/Hct at birth. After birth, lungs work → massive RBC destruction in first weeks. RBCs release heme → hemeoxygenase → biliverdin → biliverdin reductase → indirect bilirubin. But neonatal UDPGT activity is only 0.1–1% of adult levels → cannot conjugate bilirubin fast enough → jaundice.
Mastitis vs Breast Abscess vs Breast Engorgement
| Mastitis | Breast Abscess | Breast Engorgement | |
|---|---|---|---|
| Laterality | Unilateral | Unilateral | Bilateral |
| Key finding | Breast tenderness, fever, erythema | Fluctuant palpable mass | Bilateral tenderness, fever |
| Cause | Staph aureus (most common) through microbreaks in areola | Undrained mastitis | Milk engorgement, typically early postpartum |
| Treatment | Dicloxacillin/oxacillin + continue breastfeeding | Incision and drainage (no antibiotics typically needed) | Supportive, continue nursing |
Breastfeeding Contraindications (High-Yield)
Active TB · HIV (in developed countries) · Active herpes lesions on breast · Chemotherapy or radiation · Active breast cancer · Maternal drug abuse · Galactosemia (infant cannot metabolize galactose in breast milk). Note: mastitis alone is NOT a contraindication — continue breastfeeding to clear the bug.
EP317
Formula Selection, Vitamin Supplementation & Neonatal Nutrition
- Formula selection by condition: Galactosemia or lactose intolerance → soy-protein formula. Cow milk or soy allergy / multiple food allergies → protein hydrolysate formula. Short gut syndrome / post-NEC extensive bowel resection → elemental amino acid formula. Default (no breastfeeding) → cow's milk-based formula
- Vitamin supplementation in breastfed infants: Vitamin K (IM at birth to prevent hemorrhagic disease of newborn) + Vitamin D (low in breast milk). Mnemonic: Kevin Durant = KD = K + D
- Cow's milk contraindicated <1 year old: Low iron + low essential fatty acids → iron deficiency anemia risk. Also: straight cow's milk (not formula) in 8-month-old → microcytic anemia on NBME = iron deficiency from cow's milk
- Goat's milk danger: Low in folate → megaloblastic anemia (↑ MCV). Also low in iron. Classic vignette: child on a farm, using goat's milk, fatigue, MCV 110, low hemoglobin
- Honey contraindicated <1 year: Risk of infantile botulism (C. botulinum spores → germinate in infant gut → toxin production). Cow's milk also contraindicated in infants for nutritional reasons
Formula Selection — Quick Reference
| Clinical Scenario | Formula Choice | Rationale |
|---|---|---|
| Galactosemia / lactose intolerance | Soy protein | No galactose/lactose |
| Cow milk allergy / soy allergy / multiple allergies | Protein hydrolysate | Proteins broken into peptides → less allergenic |
| Short gut syndrome (NEC, extensive resection) | Elemental amino acid formula | No digestion needed — absorbed directly |
| Default (unable to breastfeed) | Cow's milk–based formula (not straight milk) | Iron-fortified, modified for infant digestion |
| Premature infant | Premature formula or fortified breast milk | Higher caloric density and minerals for growth |
Breastfeeding Benefits — Prolactin Mechanism
Prolonged breastfeeding → prolonged hyperprolactinemia → prolactin suppresses GnRH → ↓ FSH/LH → ↓ estrogen/progesterone → (1) fewer ovulations → ↓ ovarian cancer risk (fewer epithelial rupture/repair cycles), (2) ↓ estrogen → ↓ ER/PR-positive breast cancer risk.
Why Cow's Milk Causes Iron Deficiency in Infants
Cow's milk is low in iron AND the casein protein in cow's milk binds dietary iron and inhibits its absorption. Additionally, whole cow's milk can cause microscopic GI blood loss in infants (cow milk protein intolerance), further depleting iron. Never give straight cow's milk to children under 12 months.
Pediatric Infections
Pediatric infectious disease questions on Step 2/3 require recognizing classic syndromes — slapped cheek rash, aplastic crisis, Kawasaki disease, epiglottitis, croup — and knowing when to escalate. Parvovirus B19 is uniquely versatile: it causes slapped cheek in children, arthralgia in adults, and aplastic crisis in hemolytic anemias. DiGeorge syndrome questions often begin with hypocalcemic seizures or T-cell immunodeficiency.
RR 85EP420
Parvovirus B19, Beckwith-Wiedemann Syndrome & Electrolyte Traps
- Parvovirus B19 — three contexts: (1) Children: slapped-cheek rash (erythema infectiosum), knee joint pain. (2) Adults (especially daycare workers, teachers): polyarthralgias without rash. (3) Hemolytic anemia patients (sickle cell, hereditary spherocytosis): aplastic crisis — virus attacks RBC precursors → no reticulocytosis response → severe anemia. Single-stranded DNA virus
- Beckwith-Wiedemann syndrome (BWS): Overgrowth disorder. Hemihypertrophy + visceromegaly + macroglossia (large tongue) + macrosomia. Pancreatic beta-cell overgrowth → hyperinsulinemia → hypoglycemia (seizures in newborns) + hypokalemia (insulin drives K⁺ into cells via Na-K ATPase). Also: hypocalcemia (insulin drives Ca²⁺ into cells)
- BWS tumor associations: Wilms tumor (nephroblastoma) + hepatoblastoma (alpha-fetoprotein marker). Check AFP in suspected hepatoblastoma. WAGR complex: Wilms + Aniridia + Genitourinary anomalies + intellectual disability (chromosome 11)
- ACE inhibitor effects: ↓ Angiotensin II → ↓ efferent arteriolar constriction → ↓ GFR → ↑ creatinine (minor bump). ↓ Aldosterone → K⁺ retention → hyperkalemia. ↓ H⁺ excretion → metabolic acidosis. Disproportionate bump in GFR with ACE inhibitor → think renal artery stenosis
- Hyperkalemia management: Symptomatic (wide QRS, peaked T-waves) → calcium gluconate (membrane stabilization) FIRST, then insulin + glucose (drives K⁺ intracellularly via Na-K ATPase), then kayexalate/furosemide for elimination
Parvovirus B19 — Three Clinical Contexts
| Patient | Presentation | Mechanism |
|---|---|---|
| Child 3–10 years | Slapped-cheek rash, low-grade fever, knee pain, rash then spreads to trunk (lacy pattern) | Immune complex deposition |
| Adult (healthcare worker, teacher) | Polyarthralgias (symmetric, migratory), no significant rash | Immune complex arthritis |
| Hemolytic anemia patient (sickle cell, spherocytosis) | Aplastic crisis: severe anemia without reticulocytosis | Virus infects BFU-E (RBC precursors) → kills reticulocyte production → aplasia |
| Immunocompromised | Chronic anemia (virus persists, cannot be cleared) | Persistent RBC precursor destruction |
| Pregnant woman (1st trimester) | Hydrops fetalis → fetal death | Fetal RBC precursor destruction → severe anemia → high-output cardiac failure |
BWS Hyperinsulinemia — Electrolyte Chain
Pancreatic beta-cell overgrowth → hyperinsulinemia → (1) glucose driven into cells → hypoglycemia → neonatal seizures; (2) K⁺ driven into cells (Na-K ATPase upregulation) → hypokalemia → prolonged QT, muscle weakness; (3) Ca²⁺ driven into cells → hypocalcemia → Chvostek/Trousseau signs, prolonged QT, seizures. On USMLE: any of these electrolyte findings in a macrosomic infant = BWS.
ACE Inhibitor — Renal Artery Stenosis Trap
Normal response to ACE inhibitor: minor creatinine bump (<0.5 mg/dL). In renal artery stenosis: efferent arteriolar constriction (via angiotensin II) was the ONLY mechanism maintaining GFR. Remove angiotensin II → GFR collapses → large creatinine bump. Rule: creatinine ↑ >30% after starting ACE inhibitor = renal artery stenosis until proven otherwise.
RR 84EP415
DiGeorge Syndrome, Pharyngeal Pouches & Pediatric Neck Masses
- DiGeorge syndrome (22q11 deletion): 3rd and 4th pharyngeal pouches (endoderm-derived) fail to form → no thymus (T-cell deficiency → opportunistic infections, e.g., PCP) + no parathyroids (↓ PTH → hypocalcemia → seizures in newborn). Absent thymic shadow on CXR = key finding. Cardiac: truncus arteriosus, tetralogy of Fallot
- Hypocalcemia on EKG: Prolonged QT interval. Same pattern: hypokalemia, hypomagnesemia. Hypercalcemia = SHORT QT (opposite). Remember: most "hypo-" electrolytes → prolonged QT. Hyponatremia does not
- Branchial cleft cyst: Lateral neck mass that does NOT move with swallowing. Derived from ectoderm (pharyngeal clefts 2–4). Results from incomplete obliteration of branchial clefts. Imaging: ultrasound for neck masses generally
- Thyroglossal duct cyst: Midline neck mass that MOVES with swallowing and tongue protrusion. Derived from endoderm (remnant of thyroglossal duct from tongue base to thyroid). Treat: Sistrunk procedure (excise cyst + hyoid bone segment)
- Craniopharyngioma: Suprasellar calcified mass (from Rathke's pouch, roof of mouth). Presents as bitemporal hemianopsia + growth failure + diabetes insipidus. "Motor oil" fluid on histology. Imaging: calcified suprasellar mass
Pharyngeal Pouch Derivatives — Embryology Table
| Structure | Germ Layer | If Absent / Abnormal |
|---|---|---|
| 3rd pharyngeal pouch | Endoderm | No thymus (inferior parathyroids also from 3rd) |
| 4th pharyngeal pouch | Endoderm | No superior parathyroids + thyroid C-cells |
| DiGeorge (both) | — | No thymus + no parathyroids → T-cell deficiency + hypocalcemia |
| Branchial cleft cysts | Ectoderm (pharyngeal clefts 2–4) | Lateral neck cyst, does not move with swallowing |
| Thyroglossal duct | Endoderm | Midline cyst, moves with swallowing |
DiGeorge — Classic USMLE Presentation
Newborn with seizures + hypocalcemia controlled with calcium repletion → 6 months old → 2 bouts of PCP pneumonia → CXR shows absent thymic shadow. That sequence = DiGeorge. Always: neonatal hypocalcemia + T-cell immunodeficiency + cardiac anomaly (truncus arteriosus or ToF) = DiGeorge until proven otherwise.
Neck Mass — Differential by Location and Movement
| Mass | Location | Moves with Swallow? | Embryologic Origin |
|---|---|---|---|
| Thyroglossal duct cyst | Midline | Yes (and with tongue protrusion) | Endoderm — thyroglossal duct remnant |
| Branchial cleft cyst | Lateral (anterior to SCM) | No | Ectoderm — branchial cleft remnant |
| Lymphadenopathy | Multiple regions | No | — |
| Thyroid nodule / goiter | Midline / anterior | Yes (rises with swallowing) | Endoderm |
Congenital & Genetic Disorders
Genetics questions on Step 2/3 are about pattern recognition — modes of inheritance, gene loci, and clinical phenotypes. The rules from EP424 provide a cognitive framework that reduces memorization: structural protein defects → autosomal dominant; enzyme defects → autosomal recessive; immunodeficiencies → X-linked recessive. The trinucleotide repeat disorders require their gene names, not just repeat sequences.
EP424
HY Rules for Remembering Modes of Inheritance
- Autosomal dominant (AD) patterns: Structural protein defects (collagen → Marfan's OI, fibrillin → Marfan's, spectrin/ankyrin → hereditary spherocytosis), membrane receptor defects (LDL receptor → familial hypercholesterolemia), hereditary cancer syndromes (most), gain-of-function mutations (achondroplasia FGFR3, Huntington's CAG)
- Autosomal recessive (AR) patterns: Enzyme defects (PKU, galactosemia, lysosomal storage diseases, urea cycle disorders), DNA repair gene defects (ataxia-telangiectasia, Bloom syndrome, xeroderma pigmentosum, Fanconi anemia). Rule: AR if enzyme is broken
- X-linked recessive (XLR) patterns: Most immunodeficiency diseases (Bruton's agammaglobulinemia, Wiskott-Aldrich, CGD, SCID-ADA), hemophilias A and B, muscular dystrophies (Duchenne, Becker), select enzyme defects (G6PD, Lesch-Nyhan HGPRT, OTC deficiency)
- X-linked dominant (XLD) — memorize the list: Fragile X syndrome (CGG repeat, FMR1 gene), Alport syndrome (COL4A5, type 4 collagen), X-linked hypophosphatemic rickets (PHEX mutation → ↑ FGF-23), Rett syndrome (MECP2 gene)
- Mitochondrial inheritance: Ragged red fibers = key histological marker. Mom affected → all kids affected. Key disorders: MELAS (stroke-like episodes + lactic acidosis), MERRF (myoclonic epilepsy + ragged red fibers), LHON (Leber's hereditary optic neuropathy)
Inheritance Pattern — Summary Framework
| Pattern | Key Categories | Classic Examples |
|---|---|---|
| Autosomal Dominant | Structural proteins, membrane receptors, hereditary cancer syndromes, gain-of-function | Marfan's (fibrillin), OI (collagen), NF1/NF2, BRCA, Li-Fraumeni, ADPKD, achondroplasia, Huntington's |
| Autosomal Recessive | Enzyme defects, DNA repair genes | PKU, galactosemia, sickle cell, CF, lysosomal storage, Ataxia-telangiectasia, Fanconi anemia |
| X-Linked Recessive | Immunodeficiencies, hemophilias, muscular dystrophies, select enzyme defects | Bruton's, SCID, CGD, Wiskott-Aldrich, Hemophilia A&B, DMD/BMD, G6PD, Lesch-Nyhan |
| X-Linked Dominant | Memorize — very few: Fragile X, Alport, XLH, Rett | Fragile X (FMR1), Alport (COL4A5), Hypophosphatemic rickets (PHEX), Rett (MECP2) |
| Mitochondrial | Maternal inheritance, ragged red fibers | MELAS, MERRF, LHON |
Why Immunodeficiencies Are X-Linked Recessive
Many immune system genes (including genes for Btk kinase in Bruton's, WASP in Wiskott-Aldrich, NADPH oxidase in CGD, and common gamma chain in SCID) reside on the X chromosome. Males with one defective X have no backup → disease. Females with one defective X are carriers (one good X remains). This explains why most primary immunodeficiencies present in boys.
EP428
X-Linked Dominant Disorders — Fragile X, Alport, Rett & XLH
- X-linked dominant pedigree rules: Affected father → 100% of daughters affected (all get bad X) but 0% of sons affected (sons get Y from dad). Affected mother → 50% of sons AND 50% of daughters affected (50/50 which X mom passes). Both parents affected → 100% daughters, 50% sons
- Fragile X syndrome: CGG trinucleotide repeat expansion → FMR1 gene methylated/silenced. X-linked dominant. Classic: long face + large jaw + large ears + macroorchidism (big testicles) + intellectual disability + ADHD + strabismus/amblyopia. Normal life expectancy
- Alport syndrome: COL4A5 (type 4 collagen) defect → X-linked dominant. Triad: sensorineural hearing loss + ocular abnormalities (lens deformity) + nephritic syndrome. Kidney biopsy (electron microscopy): basket-weave pattern of GBM. Treat proteinuria with ACE inhibitors
- Rett syndrome: MECP2 gene mutation → X-linked dominant. Female only (males die in utero or very early). After 6 months of normal development → motor regression, hand-wringing stereotypies, speech loss, seizures, small head. Treatment: symptomatic (anticonvulsants for seizures)
- X-linked hypophosphatemia (XLH): PHEX gene mutation → ↑ FGF-23 → (1) inhibits renal phosphate reabsorption → hypophosphatemia, (2) inhibits 1α-hydroxylation of calcidiol → ↓ active vitamin D. Result: vitamin D–resistant rickets. Give phosphate supplementation + active vitamin D (calcitriol)
X-Linked Dominant Pedigree — All Permutations
| Parents | Daughters Affected | Sons Affected |
|---|---|---|
| Affected dad only | 100% (all get bad X from dad) | 0% (sons get Y from dad) |
| Affected mom only (heterozygous) | 50% | 50% |
| Both affected (dad affected + mom heterozygous) | 100% | 50% |
XLH — FGF-23 Pathophysiology
PHEX gene → normally degrades FGF-23. Broken PHEX → FGF-23 accumulates → FGF-23 acts on kidney: (1) blocks sodium-phosphate cotransporter → phosphaturia → hypophosphatemia; (2) blocks 1α-hydroxylase → cannot convert 25-OH vitamin D → 1,25-dihydroxy vitamin D → vitamin D deficiency despite normal sun exposure. Treatment: phosphate supplements + calcitriol (bypass the 1α-hydroxylase defect).
Fragile X — Full Phenotype
- Macroorchidism (large testicles) — most specific finding in post-pubertal males
- Long face, large mandible, prominent ears
- Intellectual disability (most common inherited cause of intellectual disability in males)
- ADHD is the most common comorbid psychiatric diagnosis
- Mitral valve prolapse (connective tissue involvement)
- Strabismus and amblyopia
- Normal life expectancy
- FMR1 gene: >200 CGG repeats = full mutation (methylated, silenced); 55–200 = premutation (passes to next generation with expansion risk)
RR 86EP423
Trinucleotide Repeats, Wilms Tumor & Pediatric Hip Disorders
- Trinucleotide repeat diseases (gene + chromosome): Fragile X = CGG, FMR1 gene, X chromosome. Friedreich's ataxia = GAA, frataxin (FXN) gene, chromosome 9. Huntington's = CAG, HTT gene, chromosome 4. Myotonic dystrophy = CTG, DMPK gene, chromosome 19
- Wilms tumor (nephroblastoma): Flank mass WITHOUT calcifications (contrast: neuroblastoma crosses midline and HAS calcifications). Hematuria + flank mass in child 2–6 years. WT1 gene, chromosome 11. Do NOT biopsy — nephrectomy = biopsy. Associations: WAGR complex, Beckwith-Wiedemann syndrome, Denys-Drash syndrome (Wilms + renal failure + pseudohermaphroditism)
- Pediatric hip disorders by age (DLS rule): D = Developmental dysplasia of hip (DDH): newborns. L = Legg-Calvé-Perthes: boys <10 years (avascular necrosis femoral head, idiopathic). S = Slipped capital femoral epiphysis (SCFE): obese teenage boys. Alphabetical order = ascending age order
- Avascular necrosis causes: Legg-Calvé-Perthes (idiopathic), sickle cell (vaso-occlusion), prolonged corticosteroids/Cushing's, scaphoid fracture (disrupts retrograde blood supply). All cause insidious hip pain worsening with activity, relieved by rest
- Mumps triad: Orchitis (testicular swelling → infertility risk) + parotitis + pancreatitis. No specific treatment (viral). Prevented by MMR vaccine. USMLE: macroorchidism + intellectual disability = Fragile X (not mumps — mumps causes orchitis acutely, not macroorchidism)
Trinucleotide Repeat Disorders — Master Table
| Disease | Repeat | Gene | Chromosome | Key Features |
|---|---|---|---|---|
| Fragile X syndrome | CGG | FMR1 | X | Macroorchidism, intellectual disability, long face, ADHD; X-linked dominant |
| Friedreich's ataxia | GAA | Frataxin (FXN) | 9 | Progressive ataxia, cardiomyopathy, diabetes, loss of deep tendon reflexes; autosomal recessive |
| Huntington's disease | CAG | HTT | 4 | Chorea, dementia, psychiatric, age 30–50 onset; autosomal dominant, gain-of-function |
| Myotonic dystrophy | CTG | DMPK | 19 | Myotonia, cataracts, testicular atrophy, cardiac conduction defects; autosomal dominant |
Wilms Tumor vs Neuroblastoma — Critical Distinction
Wilms tumor (nephroblastoma): Flank mass, does NOT cross midline, NO calcifications. Age 2–6. Chromosome 11 (WT1). Neuroblastoma: Crosses midline (from adrenal/para-aortic ganglia), calcifications present, elevated urine catecholamines (VMA, HVA). Different treatment and prognosis.
Pediatric Hip Disorders — Age-Based DLS Framework
| Disorder | Age Group | Key Feature | Treatment |
|---|---|---|---|
| DDH (Developmental dysplasia of hip) | Newborn | Barlow/Ortolani maneuver positive; female > male | Pavlik harness |
| Legg-Calvé-Perthes | Boys <10 (mean age 6) | Avascular necrosis femoral head, insidious hip pain with limp, worse with activity | Bracing/casting; surgery if severe |
| SCFE | Obese adolescents (>10 years) | Hip/knee pain, external rotation deformity, restricted internal rotation | Surgical fixation (screw in situ) |
Pediatric Emergencies
Child abuse and osteogenesis imperfecta sit next to each other in any pediatric emergency curriculum — they both present with multiple fractures and the key discriminator is the blue sclera of OI. SVT in children follows the same algorithm as adults: vagal maneuvers → adenosine → synchronized cardioversion if unstable. The USMLE heavily tests the arrhythmia recognition and management sequence in pediatric contexts.
RR 93EP449
Child Abuse, Osteogenesis Imperfecta & Achondroplasia
- Child abuse — recognition triggers: Multiple fractures with minimal trauma, retinal hemorrhages (shaken baby syndrome), subdural hematoma in infant, cigarette-burn marks, delayed presentation to healthcare, teenage parents, military returnee, unstable home, Munchausen by proxy (parent simulates illness). Action: call Child Protective Services immediately
- Classic fracture patterns suggesting abuse: Spiral fractures (high torsional force), posterior rib fractures (from squeezing), sternal fractures, multiple fractures at different healing stages
- Osteogenesis imperfecta (OI): Type 1 collagen defect (autosomal dominant). Multiple fragility fractures + BLUE SCLERA (thin sclera makes choroidal vessels visible) + conductive hearing loss (ossicle damage). Involved in both endochondral AND intramembranous bone formation. May need C-section delivery
- OI vs child abuse: Blue sclera = OI, not abuse. Achondroplasia: frontal bossing + short stature + bow legs (FGFR3 gain-of-function, chromosome 4). Achondroplasia = endochondral bone formation problem ONLY (contrast: OI = endochondral + intramembranous)
- Sporadic mutations (not heritable): Achondroplasia (most cases sporadic), CVID (common variable immunodeficiency), PNH (paroxysmal nocturnal hemoglobinuria — PIGA mutation). Having the disease does not mean the parent had it
OI vs Achondroplasia vs Child Abuse — Key Comparisons
| OI | Achondroplasia | Child Abuse | |
|---|---|---|---|
| Gene | COL1A1/COL1A2 (type 1 collagen) | FGFR3 (gain-of-function) | — |
| Inheritance | Autosomal dominant | Autosomal dominant (many sporadic) | — |
| Chromosome | 17 (COL1A1), 7 (COL1A2) | 4 | — |
| Key finding | Blue sclera, hearing loss | Frontal bossing, short limbs, bow legs | Retinal hemorrhage, bruising patterns |
| Bone formation | Endochondral + intramembranous | Endochondral only | — |
| Delivery | C-section recommended | C-section (for macrocephaly) | — |
Child Abuse — Mandatory Action on USMLE
ANY suspicion of child abuse → mandatory reporting to Child Protective Services (CPS). This applies regardless of: certainty of diagnosis, parental consent, or whether the child's medical needs are being addressed. Munchausen by proxy (factitious disorder imposed on another) also requires CPS report and psychiatric evaluation of the parent.
Blue Sclera — Mechanism in OI
Type 1 collagen is present throughout the body — bones, skin, sclera, tendons, teeth (dentinogenesis imperfecta). In OI, the sclera is abnormally thin due to deficient collagen. The underlying choroidal vessel layer (normally invisible) becomes visible through the transparent thin sclera → blue color. NOT blue pigment deposited. This is pathognomonic for OI.
RR 93EP449
SVT in Children, Urine pH, RTAs & Kidney Stones
- SVT in children: HR 200–300+ bpm, narrow QRS, regular rhythm. Stable → vagal maneuvers first, then adenosine. Unstable (altered, hypotensive) → synchronized cardioversion immediately. Concept applies equally to children and adults. Do NOT defibrillate SVT — defibrillation = VFib only (or pulseless VTach)
- RTA — type differentiation: Type 1 (distal, α-intercalated cell failure): urine pH >5.5, hypokalemia, nephrolithiasis (calcium phosphate stones). Type 2 (proximal, bicarb wasting): urine pH normal (<5.5), hypokalemia, Fanconi syndrome. Type 4 (low aldosterone): urine pH normal, HYPERKALEMIA (only RTA with high K⁺)
- Basic urine pH causes (urine pH >5.5): Type 1 RTA (distal tubule H⁺ pump failure) OR urease-positive organisms (Proteus mirabilis, Staph saprophyticus → ammonium magnesium phosphate = staghorn calculi)
- Kidney stone imaging: Non-contrast CT abdomen (NOT contrast CT — contrast obscures stone). Also called helical CT on exams. Hematuria = true RBCs on microscopy (contrast with myoglobinuria/rhabdomyolysis where urinalysis shows 3+ blood but <5 RBCs on microscopy)
- Renal tubular acidosis (USMLE summary): All RTAs = normal anion gap (hyperchloremic) metabolic acidosis. Differentiating factor: urine pH (type 1 = high) + potassium (type 4 = high, types 1 and 2 = low)
Renal Tubular Acidosis — Master Comparison
| Type 1 (Distal) | Type 2 (Proximal) | Type 4 (Low Aldosterone) | |
|---|---|---|---|
| Defect | α-intercalated cell H⁺ pump failure | Proximal HCO₃⁻ reabsorption failure | Aldosterone deficiency or resistance |
| Urine pH | >5.5 (cannot acidify) | <5.5 (distal cells compensate) | <5.5 |
| Serum K⁺ | Low (hypokalemia) | Low (hypokalemia) | HIGH (hyperkalemia) |
| Anion gap | Normal (hyperchloremic) | Normal | Normal |
| Common causes | Sjögren's, SLE, amphotericin B | Acetazolamide, Fanconi syndrome, multiple myeloma | Addison's, ACE inhibitors, spironolactone, diabetes |
| Complications | Nephrolithiasis (calcium phosphate), nephrocalcinosis | Osteomalacia (bicarb wasting → rickets) | Arrhythmias from hyperkalemia |
Urine vs Blood in Urinalysis — Rhabdomyolysis Trap
Urinalysis "3+ blood" does NOT always mean actual red blood cells. In rhabdomyolysis: myoglobin (a heme-containing protein like hemoglobin) causes false-positive blood on urinalysis dipstick. On urine microscopy: 0–5 RBCs per HPF. In true hematuria (kidney stone, Wilms tumor): 3+ blood on dipstick AND 30–80 RBCs per HPF on microscopy. Always check microscopy when the dipstick doesn't match the clinical picture.
Arrhythmia — Shock Type by Rhythm
| Arrhythmia | Stable | Unstable |
|---|---|---|
| SVT (narrow complex, regular) | Vagal → adenosine → beta-blocker/CCB | Synchronized cardioversion |
| Atrial flutter / AFib | Rate control → rhythm control | Synchronized cardioversion |
| VTach (wide complex, pulse present) | Amiodarone IV | Synchronized cardioversion |
| VFib / pulseless VTach | — (always pulseless) | Defibrillation (unsynchronized) |
| Asystole / PEA | — (always pulseless) | CPR + epinephrine (NO shock) |