A 6-month-old boy is brought to the emergency department by his mother because of recurrent vomiting and yellowing of his eyes. The mother says that he has been eating poorly since she started weaning him off of breast milk 5 days ago. At this time, mashed vegetables and fruits were added to his diet. Examination shows scleral jaundice and dry mucous membranes. The tip of the liver is palpable 4 cm below the right costal margin. His serum glucose concentration is 47 mg/dL, serum alanine aminotransferase is 55 U/L, and serum aspartate aminotransferase is 66 U/L. Which of the following enzymes is most likely deficient?

Galactose-1 phosphate uridyltransferase

A 22-year-old man comes to the physician for a routine physical examination. He feels well. He has no history of major medical illness and takes no medications. His vital signs are within normal limits. Examination shows no abnormalities. Laboratory studies, including a complete blood count and a standard electrolyte panel, are within normal limits. Urine dipstick is negative for glucose; a reducing substance test result of the urine is positive. Which of the following is the most appropriate dietary recommendation for this patient?

Eliminate galactose and lactose

Increase intake of ketogenic amino acids

A previously healthy 20-year-old woman comes to the physician because of recurrent abdominal cramps, bloating, and diarrhea for 4 months. She describes her stools as greasy, foul-smelling, and difficult to flush. During this time she has had a 6-kg (13.2-lb) weight loss. She has no personal or family history of serious illness. Physical examination shows pallor and cheilitis. Laboratory studies show a hemoglobin concentration of 11 g/dL. Serum concentrations of electrolytes, urea nitrogen, and creatinine are within the reference range. Test of the stool for occult blood is negative and stool microscopy reveals no pathogens and no leukocytes. Analysis of a 24-hour stool sample shows 12 g of fat. The patient is asked to consume 25 g of d-xylose. Five hours later, its concentration is measured in urine at 2 g (N = > 4 g/5 h). The test is repeated after a two-week course of rifaximin, but the urinary concentration of d-xylose remains the same. Which of the following is the most likely diagnosis?

Exocrine pancreatic insufficiency

Bacterial overgrowth in the small intestine

A 4-week-old infant is brought to the physician by his mother because of blood-tinged stools for 3 days. He has also been passing whitish mucoid strings with the stools during this period. He was delivered at 38 weeks' gestation by lower segment transverse cesarean section because of a nonreassuring fetal heart rate. He was monitored in the intensive care unit for a day prior to being discharged. His 6-year-old brother was treated for viral gastroenteritis one week ago. The patient is exclusively breastfed. He is at the 50th percentile for height and 60th percentile for weight. He appears healthy and active. His vital signs are within normal limits. Examination shows a soft and nontender abdomen. The liver is palpated just below the right costal margin. The remainder of the examination shows no abnormalities. Test of the stool for occult blood is positive. A complete blood count and serum concentrations of electrolytes and creatinine are within the reference range. Which of the following is the most appropriate next step in management?

Continue breastfeeding and advise mother to avoid dairy and soy products

Perform stool antigen immunoassay

Perform an air enema on the infant

Assess for IgA (anti‑)tissue transglutaminase antibodies (tTG)

Stop breastfeeding and switch to soy-based formula

A 15-month-old girl is brought to the pediatrician by her mother with a history of 3 episodes of breath-holding spells. The patient’s mother says that this is a new behavior and she is concerned. The patient was born at full term by spontaneous vaginal delivery with an uneventful perinatal period. She is also up to date on her vaccines. However, after the age of 6 months, the patient’s mother noticed that she was not as playful as other children of similar age. She is also not interested in interacting with others and her eye contact is poor. Her growth charts suggest that her weight, length, and head circumference were normal at birth, but there have been noticeable decelerations in weight and head circumference. On physical examination, her vital signs are normal. A neurologic examination reveals the presence of generalized mild hypotonia. She also makes repetitive hand wringing motions. Which of the following clinical features is most likely to develop in this patient during the next few years?

Loss of purposeful use of her hands

A 16-year-old boy is brought to the physician for a follow-up examination. He has a 6-year history of type 1 diabetes mellitus and his only medication is insulin. Seven months ago, he was treated for an episode of diabetic ketoacidosis. He has previously been compliant with his diet and insulin regimen. He wants to join the high school soccer team. Vital signs are within normal limits. His hemoglobin A1C is 6.3%. Which of the following is the most appropriate recommendation at this time?

Lower insulin dosage on days of exercise

Limit activity to 20 minutes per day

Advise against physical activity

Switch from insulin to metformin

Increase insulin dosage on days of exercise

Special diets in pediatric conditions for USMLE Step 1: High-Yield Pediatrics Lessons

Amino Acid Disorders - Protein Problems

Phenylketonuria (PKU):
- Deficiency: Phenylalanine hydroxylase.
- Diet: Lifelong low-phenylalanine diet. Avoid high-protein foods (meat, dairy) & aspartame.
- Formula: Phenylalanine-free formulas (e.g., Lofenalac, Phenyl-Free).
Maple Syrup Urine Disease (MSUD):
- Defect: Branched-chain α-ketoacid dehydrogenase.
- Diet: Restriction of branched-chain amino acids (BCAAs) - Leucine, Isoleucine, Valine.
- Urine smells like burnt sugar/maple syrup.
Homocystinuria:
- Classic type: Cystathionine β-synthase deficiency.
- Diet: Methionine-restricted, cysteine-supplemented.
- Treatment: High-dose Vitamin B6 (pyridoxine) for responsive forms. Folate & B12 supplement.

⭐ In PKU, a characteristic musty or mousy body odor is present due to phenylacetate accumulation.

Carbohydrate Disorders - Sugar Shock

Galactosemia (GALT Deficiency):
- Neonatal onset: Jaundice, hepatomegaly, vomiting, cataracts.
- Dx: ↑ Galactose-1-P in RBCs; non-glucose reducing substances in urine.
- Rx: Lifelong lactose-free diet (e.g., soy formula).
Hereditary Fructose Intolerance (Aldolase B Deficiency):
- Onset after introducing fruits/sucrose.
- Features: Hypoglycemia, vomiting, hepatomegaly, aversion to sweets.
- Rx: Lifelong fructose & sucrose-free diet.
Key Glycogen Storage Diseases (GSDs):
- Type I (von Gierke): Severe fasting hypoglycemia, lactic acidosis. Rx: Uncooked cornstarch.
- Type II (Pompe): Cardiomegaly, profound hypotonia ("floppy infant").
- Type V (McArdle): Exercise intolerance, cramps, myoglobinuria.

⭐ Galactosemia is classically associated with neonatal E. coli sepsis.

Galactosemia metabolic pathway and GALT enzyme block

Malabsorption Syndromes - Gut Grief

Core Issue: Impaired intestinal absorption of nutrients.
Presentation: Chronic diarrhea, steatorrhea, weight loss, nutritional deficiencies.

Celiac Disease: Normal vs. Atrophic Villous Histology

Condition	Key Pathophysiology	Dietary Management
Celiac Disease	Gluten-sensitive enteropathy → villous atrophy	Lifelong gluten-free diet (No wheat, barley, rye)
Lactose Intolerance	Lactase enzyme deficiency	Avoidance or reduction of dairy products
Tropical Sprue	Post-infectious; affects tropical residents/visitors	Folic acid + Tetracycline for 3-6 months
Whipple's Disease	Tropheryma whipplei infection	Prolonged antibiotics (not primarily diet)

Therapeutic & Elimination Diets - Fuel-Switch Feeds

Ketogenic Diet (KD): High-fat, adequate-protein, low-carbohydrate diet that forces the body to burn fats rather than carbohydrates.
- Indication: Refractory epilepsy, GLUT-1 deficiency, Pyruvate Dehydrogenase Complex deficiency.
- Types: Classic 4:1 (Fat:CHO+Protein), MCT oil-based, Modified Atkins Diet (MAD), Low Glycemic Index Treatment (LGIT).
- Mechanism: Shifts metabolism from glycolysis to ketosis, producing ketone bodies (β-hydroxybutyrate) as an alternative fuel for the brain.

⭐ The classic ketogenic diet maintains a strict 4:1 ratio by weight of fat to combined protein and carbohydrate, inducing a state of ketosis.

Elimination Diets: Used for diagnosis & management of food allergies (e.g., Cow's Milk Protein Allergy - CMPA).
- Involves removing the suspected allergen, followed by a planned re-challenge to confirm.

Ketogenic Diet Macronutrient Breakdown

High‑Yield Points - ⚡ Biggest Takeaways

PKU requires a low-phenylalanine diet; avoid high-protein foods.

Galactosemia mandates a strict lactose-free and galactose-free diet; use soy formula.

MSUD treatment involves restricting branched-chain amino acids (leucine, isoleucine, valine).

Celiac disease necessitates a lifelong gluten-free diet (avoid wheat, barley, rye).

GSD management focuses on frequent feeds and uncooked cornstarch to prevent hypoglycemia.

Wilson's disease requires a low-copper diet, avoiding nuts and shellfish.

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