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AMC CAT MCQ Examination (AMC Part 1)

Faltering growth and developmental comparisons

Q: A 9-month-old infant is evaluated for faltering growth. He was born at full term with birth weight, length, and head circumference all tracking on the 50th percentile. He was exclusively breastfed until 6 months of age, during which time his growth tracked steadily along the 50th percentile. Over the past 3 months following the introduction of solid weaning foods including infant semolina, wheat cereals, and bread crusts, his weight has fallen steeply, crossing from the 50th percentile down to below the 3rd percentile. His length now tracks on the 25th percentile, while head circumference remains preserved on the 50th percentile. His mother reports that he has become increasingly irritable, has lost interest in playing, and passes four to five pale, bulky, foul-smelling, loose stools daily. Physical examination reveals a protuberant abdomen, wasted gluteal musculature, and generalized pallor. Which of the following represents the most likely diagnosis and first-line diagnostic investigation?

Suspected coeliac disease; anti-tTG IgA plus total IgA followed by the paediatric diagnostic pathway. The gluten-related symptoms and faltering growth support coeliac assessment with anti-tTG IgA and total IgA. Serology is not universal stand-alone confirmation; paediatric no-biopsy diagnosis has strict specialist criteria, otherwise further assessment is needed. Keep gluten until testing is complete. CF can present beyond birth, so the history does not categorically exclude it.

Key Takeaways

Serial measurements are more informative than a single low percentile.
Observe a feed when feeding technique or milk transfer may contribute.
Persistent vomiting, systemic illness or developmental concerns require further investigation.

Last updated: October 2026

Growth Charts & Failure to Thrive (Faltering Growth)

WHO Growth Charts

Australian guidance commonly recommends the World Health Organization (WHO) Child Growth Standards (0 to 2 years) based on healthy, breastfed infants living in optimal environments. Beyond 2 years, either WHO or Centers for Disease Control (CDC) reference charts are utilized.

Parameters: Weight-for-age, Length/height-for-age, Head circumference-for-age, Weight-for-length/height, and Body Mass Index (BMI)-for-age.
Prematurity Correction: Growth parameters for infants born at $< 37\text{ weeks}$ gestation must be plotted using corrected age (Chronological Age minus Weeks of Prematurity) until at least 2 years of chronological age (or up to 3 years for severe prematurity $< 28\text{ weeks}$ ).

Definition of Failure to Thrive (FTT)

Failure to thrive (better termed "faltering growth") is a physical sign of undernutrition, not a disease entity. It is formally defined by any of the following:

A fall in weight across two or more major percentile lines (e.g., 50th centile $\rightarrow$ 10th centile $\rightarrow$ $< 3$ rd centile) on standard growth charts.
Weight persistently below the 3rd percentile.
Weight-for-length/height below the 5th percentile.

Pattern of Deprivation: Weight falters first (acute undernutrition). If undernutrition is chronic, linear height/length decelerates (stunting). Head circumference is preserved until late, severe malnutrition (microcephaly or "head-sparing").

Diagnostic Evaluation & Refeeding Principles

History & Feeding Observation: A meticulous 3 to 7-day food diary. Observe feeding directly in the clinic: examine maternal-infant attachment, bottle preparation (checking scoop-to-water ratios to exclude over-dilution), latch, and infant behavioural responses during meals.
Minimal Targeted Laboratory Screening: Extensive baseline investigations have a diagnostic yield $< 1\%$ $< 1%$ in the absence of clinical signs. Initial targeted screen includes:
- Full blood count and ferritin (iron deficiency anaemia).
- Urea, electrolytes, creatinine, and venous bicarbonate (evaluating for Renal Tubular Acidosis—characterized by normal anion gap metabolic acidosis, hypokalaemia, and faltering growth).
- Coeliac serology: Anti-tissue transglutaminase IgA (anti-tTG IgA) along with total serum IgA (screening is valid only after dietary introduction of gluten, typically after 6 months of age; selective IgA deficiency necessitates IgG-based testing).
- Sweat chloride test by pilocarpine iontophoresis (Cystic Fibrosis if recurrent respiratory infections, steatorrhoea, or elevated IRT on bloodspot).
- Urine microscopy and culture (occult urinary tract infection).
Nutritional Rehabilitation:
- Involve a paediatric dietitian and community child health nurse.
- Provide $120\%\text{ to }150\%$ of the recommended daily caloric intake for age based on ideal weight for height.
- Increase caloric density (e.g., concentrating infant formula under dietetic guidance, adding glucose polymers or vegetable oils to solid foods, high-calorie oral nutritional supplements).
- Monitor for refeeding syndrome in severely malnourished infants.
Faltering growth: Compare reliable serial measurements and feeding history, assess medical and social causes, and provide feeding support. Improvement in hospital does not by itself prove a non-medical cause or caregiver neglect. Children below the third centile may be constitutionally small; trajectory and clinical condition matter.

Clinical comparison: Comprehensive Checkpoint Developmental Milestones and Red Flags

6 to 8 Weeks: Gross Motor Milestone: Head in line in ventral suspension; lifts chin prone; Fine Motor & Vision Milestone: Fixes and follows $90^\circ$ ; hands predominantly closed; Speech & Language Milestone: Startles to loud noises; coos and gurgles; Social & Emotional Milestone: Responsive social smile; Absolute Red Flag (Urgent Action): No social smile by 8 weeks; no eye contact
6 Months: Gross Motor Milestone: Rolls over; sits propped on hands (tripod); Fine Motor & Vision Milestone: Reaches with both hands; transfers objects hand-to-hand; Speech & Language Milestone: Polysyllabic babble ("ba-ba", "da-da"); turns to voice; Social & Emotional Milestone: Stranger anxiety; laughs; plays peek-a-boo; Absolute Red Flag (Urgent Action): Persistent primitive reflexes; no reaching; no vocal sounds
9 Months: Gross Motor Milestone: Sits unsupported with straight back; crawls; Fine Motor & Vision Milestone: Inferior pincer grasp; pokes with index finger; Speech & Language Milestone: Understands "no"; responds to own name; Social & Emotional Milestone: Waves "bye-bye"; plays "pat-a-cake"; Absolute Red Flag (Urgent Action): Cannot sit unsupported by 9 months; no babble
12 Months: Gross Motor Milestone: Stands briefly; cruises along furniture; Fine Motor & Vision Milestone: Mature pincer grasp; points with index finger; Speech & Language Milestone: 1 to 2 words with meaning; follows gesture commands; Social & Emotional Milestone: Drinks from cup held by adult; cooperative dressing; Absolute Red Flag (Urgent Action): No mature pincer grasp; no pointing; no words
18 Months: Gross Motor Milestone: Walks well independently; runs stiffly; Fine Motor & Vision Milestone: Tower of 3 to 4 cubes; spontaneous scribbles; Speech & Language Milestone: Expressive vocabulary of 10 to 20 words; Social & Emotional Milestone: Uses spoon; imitates domestic chores; Absolute Red Flag (Urgent Action): Cannot walk independently; $< 10$ words; check CK
2 Years: Gross Motor Milestone: Runs well; kicks ball; jumps with both feet; Fine Motor & Vision Milestone: Tower of 6 cubes; copies vertical line; Speech & Language Milestone: Combines words into 2-word phrases; $> 50$ words; Social & Emotional Milestone: Parallel play; takes off simple clothing; Absolute Red Flag (Urgent Action): No 2-word phrases; speech $< 50\%$ intelligible
3 Years: Gross Motor Milestone: Pedals tricycle; alternates feet up stairs; Fine Motor & Vision Milestone: Tower of 9 cubes; copies circle; uses scissors; Speech & Language Milestone: 3 to 4-word sentences; asks "why?"; $75\%$ intelligible; Social & Emotional Milestone: Interactive cooperative play; daytime toilet trained; Absolute Red Flag (Urgent Action): Speech $< 75\%$ intelligible; cannot copy circle
Drawing: A cross is a typical four-year task; copying a square is commonly a five-year skill. Milestone ranges and opportunity to practise matter; one drawing does not prove pathology.

Primary references (checked 7 October 2026): Australian autism diagnosis guideline.

Test Your Knowledge

A 9-month-old infant is evaluated for faltering growth. He was born at full term with birth weight, length, and head circumference all tracking on the 50th percentile. He was exclusively breastfed until 6 months of age, during which time his growth tracked steadily along the 50th percentile. Over the past 3 months following the introduction of solid weaning foods including infant semolina, wheat cereals, and bread crusts, his weight has fallen steeply, crossing from the 50th percentile down to below the 3rd percentile. His length now tracks on the 25th percentile, while head circumference remains preserved on the 50th percentile. His mother reports that he has become increasingly irritable, has lost interest in playing, and passes four to five pale, bulky, foul-smelling, loose stools daily. Physical examination reveals a protuberant abdomen, wasted gluteal musculature, and generalized pallor. Which of the following represents the most likely diagnosis and first-line diagnostic investigation?

Congenital hypothyroidism diagnosed by elevated thyroid-stimulating hormone and low free thyroxine

Renal tubular acidosis identified by normal anion gap metabolic acidosis on arterial blood gas analysis

Cystic fibrosis diagnosed by elevated stool elastase concentration and genetic mutation sequencing

Suspected coeliac disease; anti-tTG IgA plus total IgA followed by the paediatric diagnostic pathway

Sections you finish are checked off in the contents.

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