A 9-month-old infant presents to your office for a check-up. Exam reveals developmental delay, microcephaly, and a mousy odor to his breath. You should be concerned that the infant may have which of the following?

Deficit of phenylalanine hydroxylase activity

Excess tetrahydrobiopterin cofactor

Deficit of porphobilinogen deaminase activity

Deficit of tyrosine hydroxylase activity

Excess phenylalanine hydroxylase activity

An investigator is studying nutritional deficiencies in humans. A group of healthy volunteers are started on a diet deficient in pantothenic acid. After 4 weeks, several of the volunteers develop irritability, abdominal cramps, and burning paresthesias of their feet. These symptoms are fully reversed after reintroduction of pantothenic acid to their diet. The function of which of the following enzymes was most likely impaired in the volunteers during the study?

Alpha-ketoglutarate dehydrogenase

A 10-day-old infant with MSUD is admitted with lethargy, poor feeding, and plasma leucine of 2000 μmol/L (normal <200). Despite BCAA-free formula, the infant's condition worsens with encephalopathy. The neonatology team proposes hemodialysis, while the genetics team suggests continuing medical management with high-calorie IV fluids. The family is concerned about invasive procedures. Evaluate the optimal management strategy.

Immediate hemodialysis to rapidly lower leucine levels

High-dose thiamine trial before considering dialysis

Continue medical management and reassess in 24 hours

Exchange transfusion with albumin solutions

Start peritoneal dialysis as less invasive alternative

A research study compares three siblings with homocystinuria: one responds to pyridoxine therapy, one responds to betaine, and one requires both treatments plus dietary restriction. All have elevated homocysteine but different methionine levels. Analyze which underlying molecular mechanism best explains the variable treatment responses among these siblings.

Different mutations in the same CBS gene causing varying residual enzyme activity

Variable dietary methionine intake affecting phenotype expression

Different epigenetic modifications of the CBS gene

Varying levels of cystathionine β-synthase inhibitors

Mutations in three different genes affecting homocysteine metabolism

A 4-year-old boy presents with dark urine that turns black upon standing. His parents report that his diapers developed dark stains since infancy. Physical examination is otherwise normal. Urine analysis shows elevated homogentisic acid. What metabolic consequence is this patient at risk for developing in adulthood?

Ochronotic arthropathy affecting spine and large joints

Progressive intellectual disability and seizures

Cardiomyopathy with heart failure

Cirrhosis with portal hypertension

Chronic kidney disease from renal stone formation

Essential vs. non-essential amino acids for USMLE Step 3: High-Yield Biochemistry Lessons

Essential Amino Acids - The VIP List

Cannot be synthesized de novo by the body; must be supplied in the diet.
📌 Mnemonic: PVT TIM HALL
- Phenylalanine
- Valine
- Threonine
- Tryptophan
- Isoleucine
- Methionine
- Histidine
- Arginine (conditionally essential)
- Leucine
- Lysine

⭐ Exam Favorite: Leucine and Lysine are exclusively ketogenic amino acids. They are the exceptions that get tested!

Non-Essential & Conditional - The Home Team

Non-Essential (NEAA): Synthesized de novo by the body.
- Alanine, Asparagine, Aspartate, Glutamate, Serine.
- 📌 Mnemonic: "Basically, All Awesome Athletes Get Stronger" (Alanine, Asparagine, Aspartate, Glutamate, Serine).
- Derived from intermediates of glycolysis or the TCA cycle.
Conditionally Essential: Synthesis can be limited under specific conditions (e.g., prematurity, severe catabolic stress).
- Arginine, Cysteine, Glutamine, Glycine, Proline, Tyrosine.
- Become essential if a precursor essential amino acid is deficient.

Synthesis of non-essential amino acids

⭐ In Phenylketonuria (PKU), a deficiency of phenylalanine hydroxylase prevents the conversion of Phenylalanine (essential) to Tyrosine. This makes Tyrosine an essential amino acid for these patients.

Metabolic Origins - The Amino Acid Factory

Non-essential amino acids (NEAAs) are synthesized endogenously, primarily from intermediates of core metabolic pathways.
Essential amino acids (EAAs) must be acquired from the diet as humans lack the necessary synthetic pathways.
- 📌 Mnemonic: PVT TIM HALL (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine).
- Arginine is often considered conditionally essential, especially in children and during illness.

Amino Acid Metabolism and Krebs Cycle Intermediates

⭐ In Phenylketonuria (PKU), a defect in phenylalanine hydroxylase blocks the conversion of Phenylalanine to Tyrosine, making Tyrosine a conditionally essential amino acid.

High‑Yield Points - ⚡ Biggest Takeaways

Essential amino acids must be obtained from the diet as the body cannot synthesize them de novo.

The 10 essential amino acids are remembered by the mnemonic "PVT TIM HALL".

Non-essential amino acids are synthesized from intermediates of glycolysis and the TCA cycle.

Arginine is semi-essential, required during periods of rapid growth or metabolic stress.

Conditionally essential amino acids like Tyrosine become essential in diseases like PKU due to precursor deficiency.

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