Growth, Failure to Thrive & Nutrition
A high-yield Step 2 CK walkthrough of normal growth parameters, the failure-to-thrive workup (weight falls first; history and observed feeding before labs), normal-variant short stature, and the classic pediatric nutritional deficiencies including marasmus vs. kwashiorkor.
Why growth is the boards' favorite vital sign
Growth is the most sensitive marker of a child's overall health, which is why the boards lean on it so heavily. The skill being tested is interpreting plotted growth parameters — weight, length/height, and head circumference — to separate benign normal variants (constitutional growth delay, familial short stature) from true pathology, and to recognize failure to thrive (FTT), in which caloric intake is inadequate. Two rules anchor most questions: know which parameter falls first (weight, because it reflects nutrition) and know the workup order (a careful feeding/diet history and observed feeding come before any lab panel). Nutrition items then cluster around single-nutrient deficiencies — iron, vitamin D, B12 — and protein-energy malnutrition.
- Newborns lose up to ~7% of birth weight in the first week (up to 10% may be acceptable; >10% warrants evaluation); regain birth weight by 10-14 days
- Double birth weight by ~4-5 months, triple by 12 months, quadruple by 24 months
- Length increases ~50% by 12 months; growth velocity ~25 cm (yr 1), ~12 cm (yr 2), then ~5-7 cm/yr until puberty
- Anterior fontanelle closes by ~18-24 mo, posterior by ~2 mo (delayed anterior closure → think hypothyroidism, rickets, ↑ICP)
- FTT = weight <3rd-5th percentile for age, OR weight-for-length <5th percentile, OR a sustained downward crossing of ≥2 major percentile lines
- Order malnutrition hits parameters: weight first → length next → head circumference last (HC is spared except in severe/chronic malnutrition)
- If length AND head circumference are proportionately small from birth, think genetic/congenital (syndrome, TORCH), not caloric deprivation
The short child — three classic profiles
| Feature | Constitutional delay | Familial short stature | Pathologic (endocrine) |
|---|---|---|---|
| Bone age | Delayed (= height age) | Normal (= chronologic age) | Delayed |
| Growth velocity | Normal | Normal | Decreased |
| Puberty | Delayed | Normal | Delayed/abnormal |
| Family clue | "Late-bloomer" parent | Short parents | None |
| Adult height | Normal (reaches target) | Short (= midparental) | Short unless treated |
| Body habitus | Thin, normal | Proportionate | Short + overweight |
Vignette: A 9-month-old born at term (birth weight 50th percentile) is now <3rd percentile for weight, 25th for length, with normal head circumference. Exam is normal. The caregiver over-dilutes the formula to "make it last," and feeding times are chaotic. During a supervised hospital admission he gains weight briskly.
Diagnosis: Non-organic (psychosocial) failure to thrive from inadequate caloric intake — the most common cause of FTT.
Next best step: Take a detailed diet/feeding history and directly observe a feeding; plot serial growth. Do not order a broad lab/imaging panel first — labs are low-yield absent a clue on history/exam. Weight gain with adequate calories in a supervised setting confirms the diagnosis. When refeeding severe malnutrition, monitor phosphate (refeeding hypophosphatemia). Note: over-diluting formula also risks hyponatremia/water intoxication → seizures.
- Iron (most common childhood nutritional deficiency): classic toddler drinking excess cow's milk (>24 oz/day) → microcytic, hypochromic anemia, ↑RDW, ↓ferritin; screen Hb at 12 mo. Whole cow's milk before 12 mo → occult GI blood loss.
- Vitamin D: exclusively breastfed infants need 400 IU/day from birth to prevent rickets (bowed legs, rachitic rosary, widened wrists, craniotabes, delayed fontanelle closure). Labs: ↓/normal Ca, ↓phosphate, ↑alk phos, ↑PTH, ↓25-OH vitamin D.
- Vitamin K: single IM dose at birth prevents hemorrhagic disease of the newborn (highest risk if breastfed).
- Vitamin B12: exclusively breastfed infant of a vegan mother → megaloblastic anemia + developmental regression/hypotonia.
- Zinc: acrodermatitis enteropathica — perioral/perianal/acral dermatitis, alopecia, diarrhea.
- Vitamin C: scurvy — bleeding swollen gums, perifollicular hemorrhage, poor healing.
- Vitamin A: night blindness, Bitot spots, xerophthalmia.
Vignette: A 15-month-old is pale and drinks ~40 oz of whole milk daily, eating few solids. Hb 8.4 g/dL, low MCV, high RDW, low ferritin.
Diagnosis: Iron-deficiency anemia from excessive cow's-milk intake — iron-poor milk displaces iron-rich solid foods (± cow's-milk-related occult GI blood loss).
Next best step: Limit cow's milk to <16-24 oz/day, add iron-rich foods, and start oral iron. A therapeutic iron trial is both diagnostic and therapeutic — expect reticulocytosis in ~1 week and a Hb rise over the following weeks. No marrow exam or endoscopy is needed for the classic dietary case.
Marasmus vs. Kwashiorkor
| Feature | Marasmus | Kwashiorkor |
|---|---|---|
| Cause | Total calorie (energy) deficit | Protein deficit, calories adequate |
| Edema | Absent | Present (hypoalbuminemia): pedal, periorbital |
| Wasting | Severe; "old-man" facies | Masked by edema |
| Subcut fat / muscle | Markedly depleted | Relatively preserved |
| Liver | Normal | Fatty hepatomegaly |
| Skin / hair | Dry, thin | "Flaky-paint" rash; hair depigmentation (flag sign) |
| Appetite | Preserved (hungry) | Poor |
"ADEK" = the fat-soluble vitamins (A, D, E, K). They require bile + pancreatic lipase to absorb, so they deplete in fat malabsorption (cystic fibrosis, cholestasis, celiac disease):
- A — night blindness, xerophthalmia, Bitot spots
- D — rickets (children) / osteomalacia; ↓Ca, ↓PO4, ↑PTH
- E — hemolytic anemia; ataxia + peripheral neuropathy (dorsal column / spinocerebellar)
- K — bleeding, ↑PT/INR; hemorrhagic disease of the newborn
Growth pearl: in malnutrition, parameters fall in order — Weight → Length → Head circumference (weight first, head circumference last).
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