An 8-month-old boy is brought to the physician by his parents for gradually increasing loss of neck control and inability to roll over for the past 2 months. During this time, he has had multiple episodes of unresponsiveness with a blank stare and fluttering of the eyelids. His parents state that he sometimes does not turn when called but gets startled by loud noises. He does not maintain eye contact. He was able to roll over from front to back at 5 months of age and has not yet begun to sit or crawl. His parents are of Ashkenazi Jewish descent. Neurological examination shows generalized hypotonia. Deep tendon reflexes are 3+ bilaterally. Plantar reflex shows extensor response bilaterally. Fundoscopy shows bright red macular spots bilaterally. The remainder of the examination shows no abnormalities. Which of the following is the most likely cause of this patient's symptoms?

β-glucocerebrosidase deficiency

ATP-binding cassette transporter mutation

An 18-month-old boy of Ashkenazi-Jewish descent presents with loss of developmental milestones. On ocular exam, a cherry-red macular spot is observed. No hepatomegaly is observed on physical exam. Microscopic exam shows lysosomes with onion-skin appearance. What is the most likely underlying biochemical abnormality?

Accumulation of GM2 ganglioside

Accumulation of ceramide trihexoside

Accumulation of glucocerebroside

Accumulation of galactocerebroside

A 27-year-old woman presents to the emergency department complaining of a left-sided headache and right-sided blurry vision. She states that 2 weeks ago she developed dark urine and abdominal pain. She thought it was a urinary tract infection so she took trimethoprim-sulfamethoxazole that she had left over. She planned on going to her primary care physician today but then she developed headache and blurry vision so she came to the emergency department. The patient states she is otherwise healthy. Her family history is significant for a brother with sickle cell trait. On physical examination, there is mild abdominal tenderness, and the liver edge is felt 4 cm below the right costal margin. Labs are drawn as below: Hemoglobin: 7.0 g/dL Platelets: 149,000/mm^3 Reticulocyte count: 5.4% Lactate dehydrogenase: 3128 U/L Total bilirubin: 2.1 mg/dL Indirect bilirubin: 1.4 mg/dL Aspartate aminotransferase: 78 U/L Alanine aminotransferase: 64 U/L A peripheral smear shows polychromasia. A Doppler ultrasound of the liver shows decreased flow in the right hepatic vein. Magnetic resonance imaging of the brain is pending. Which of the following tests, if performed, would most likely identify the patient’s diagnosis?

Glucose-6-phosphate-dehydrogenase levels

An 18-month-old girl is brought to the pediatrician’s office for failure to thrive and developmental delay. The patient’s mother says she has not started speaking and is just now starting to pull herself up to standing position. Furthermore, her movement appears to be restricted. Physical examination reveals coarse facial features and restricted joint mobility. Laboratory studies show increased plasma levels of several enzymes. Which of the following is the underlying biochemical defect in this patient?

Failure of mannose phosphorylation

Congenital lack of lysosomal formation

Inappropriate protein targeting to endoplasmic reticulum

Inappropriate degradation of lysosomal enzymes

A 2-year-old boy is brought to the physician by his mother for evaluation of recurrent infections and easy bruising. He has been hospitalized 3 times for severe skin and respiratory infections, which responded to treatment with antibiotics. Examination shows sparse silvery hair. The skin is hypopigmented and there are diffuse petechiae. Laboratory studies show a hemoglobin concentration of 8 g/dL, leukocyte count of 3000/mm3, and platelet count of 45,000/mm3. A peripheral blood smear shows giant cytoplasmic granules in granulocytes and platelets. Which of the following is the most likely underlying cause of this patient's symptoms?

Defective lysosomal trafficking regulator gene

Lysosomal enzyme deficiencies for USMLE Step 3: High-Yield Biochemistry Lessons

LSDs - Cellular Trash Cans

Core Defect: Inherited mutations causing deficient or absent lysosomal enzymes, the cell's recycling centers.
Mechanism: Failure to degrade complex macromolecules (e.g., sphingolipids, mucopolysaccharides).
Pathology: Leads to toxic accumulation of undigested substrates within lysosomes, causing cellular swelling and progressive organ damage (hepatosplenomegaly, neurodegeneration).

⭐ Most LSDs are autosomal recessive. Key exceptions are Fabry disease (X-linked recessive) and Hunter syndrome (X-linked recessive).

Sphingolipidoses - Fatty Buildup

Inherited deficiencies of enzymes for sphingolipid metabolism.
Most are autosomal recessive, except for Fabry disease (X-linked recessive).

Disease	Deficient Enzyme	Accumulated Substrate	Key Features
Tay-Sachs	Hexosaminidase A	$GM2$ ganglioside	Cherry-red spot; NO hepatosplenomegaly; onion skin lysosomes.
Fabry	α-galactosidase A	Ceramide trihexoside	Peripheral neuropathy; angiokeratomas; renal failure.
Metachromatic Leukodystrophy	Arylsulfatase A	Cerebroside sulfate	Central & peripheral demyelination; ataxia; dementia.
Krabbe	Galactocerebrosidase	Galactocerebroside	Peripheral neuropathy; optic atrophy; globoid cells.
Gaucher	Glucocerebrosidase	Glucocerebroside	Hepatosplenomegaly; pancytopenia; bone crises; crumpled tissue paper cells.
Niemann-Pick	Sphingomyelinase	Sphingomyelin	Hepatosplenomegaly; cherry-red spot; foam cells.

⭐ Gaucher disease is the most common lysosomal storage disease, characterized by "crumpled tissue paper" macrophages on bone marrow biopsy.

📌 Mnemonic: No man picks his nose with his sphinger (Niemann-Pick, Sphingomyelinase).

Mucopolysaccharidoses - Sugar Mess

Group of inherited metabolic disorders caused by the absence or malfunctioning of lysosomal enzymes needed to break down glycosaminoglycans (GAGs).
Hurler Syndrome (MPS I)
- Deficiency: α-L-iduronidase.
- Accumulation: Heparan & dermatan sulfate.
- Features: Corneal clouding, gargoylism, dysostosis multiplex, developmental delay.
- Inheritance: Autosomal Recessive.
Hunter Syndrome (MPS II)
- Deficiency: Iduronate-2-sulfatase.
- Accumulation: Heparan & dermatan sulfate.
- Features: No corneal clouding, aggressive behavior, mild to severe physical & intellectual disability.

⭐ Hunter syndrome is X-linked recessive, a key distinguishing feature from most other autosomal recessive lysosomal storage diseases.

📌 Mnemonic: Hunters see clearly (no corneal clouding) and aim for the X (X-linked).

Hurler syndrome facial features

Diagnosis & Management - Lab Sleuthing

Primary Test: Direct measurement of specific enzyme activity in leukocytes or cultured fibroblasts is the gold standard.
Confirmatory: Genetic testing for specific gene mutations confirms the diagnosis and is vital for carrier detection and prenatal counseling.
Management Principles:
- Enzyme Replacement Therapy (ERT): Mainstay for many LSDs (e.g., Gaucher, Fabry).
- Substrate Reduction Therapy (SRT): Inhibits synthesis of the accumulating substrate.
- Hematopoietic Stem Cell Transplant (HSCT): For certain mucopolysaccharidoses (e.g., Hurler syndrome).

⭐ Enzyme Replacement Therapy (ERT) infusions do not cross the blood-brain barrier, limiting efficacy for neurological symptoms.

High‑Yield Points - ⚡ Biggest Takeaways

Most are autosomal recessive, except for the X-linked Fabry and Hunter diseases.

Tay-Sachs and Niemann-Pick both present with a "cherry-red" macula, but hepatosplenomegaly is absent in Tay-Sachs.

Gaucher disease is the most common, featuring massive hepatosplenomegaly and "crinkled paper" macrophages.

Fabry disease is characterized by peripheral neuropathy, angiokeratomas, and eventual renal failure.

Hunter syndrome (X-linked) is distinguished from Hurler by the absence of corneal clouding and presence of aggressive behavior.

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