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

A 24-year-old man comes to the physician because of chronic fatigue and generalized weakness after exertion. His legs feel stiff after walking long distances and he has leg cramps after climbing stairs. His symptoms are always relieved by rest. Urine dipstick shows 3+ blood and urinalysis is negative for RBCs. Baseline venous lactate and serum ammonia levels are collected, after which a blood pressure cuff is attached to the upper right arm. The patient is asked to continuously pump his right arm with the cuff inflated and additional venous samples are collected at 2-minute intervals. Analysis of the venous blood samples shows that, over time, serum ammonia levels increase and venous lactate levels remain stable. A biopsy of the right gastrocnemius muscle will most likely show which of the following?

Subsarcolemmal periodic acid–Schiff-positive deposits

Intrafascicular CD8+ lymphocytic infiltration

Endomysial fibrosis with absent dystrophin

Intermyofibrillar proliferation of mitochondria

Perivascular CD4+ lymphocytic infiltrate

Lipid storage diseases for USMLE Step 1: High-Yield Biochemistry Lessons

Sphingolipidoses - The Lipid Saboteurs

Inherited lysosomal storage disorders caused by defects in sphingolipid metabolism, leading to accumulation in various tissues.

Disease	Deficient Enzyme	Accumulated Substrate	Key Features
Tay-Sachs	Hexosaminidase A	GM2 ganglioside	Cherry-red macula, neurodegeneration, onion skin lysosomes. NO hepatosplenomegaly.
Fabry	α-galactosidase A	Ceramide trihexoside	X-linked. Peripheral neuropathy, angiokeratomas, renal failure.
Gaucher	Glucocerebrosidase	Glucocerebroside	Hepatosplenomegaly, pancytopenia, bone crises.
Niemann-Pick	Sphingomyelinase	Sphingomyelin	Neurodegeneration, hepatosplenomegaly, cherry-red macula, foam cells.

⭐ Gaucher cells are lipid-laden macrophages resembling crumpled tissue paper.

📌 Mnemonic: Tay-Sachs has Spotted macula. Fabry's is Famously X-rated (X-linked).

Key Diseases - The Enzyme Defect Lineup

Tay-Sachs Disease
- Enzyme: Hexosaminidase A
- Substrate: GM2 ganglioside
- 📌 Mnemonic: "Tay-SaX lacks heXosaminidase A"
Fabry Disease
- Enzyme: α-galactosidase A
- Substrate: Ceramide trihexoside
- 📌 Mnemonic: "My FABRYte activity is making CERAMICs"
Metachromatic Leukodystrophy
- Enzyme: Arylsulfatase A
- Substrate: Cerebroside sulfate
Krabbe Disease
- Enzyme: Galactocerebrosidase
- Substrate: Galactocerebroside, psychosine
Gaucher Disease
- Enzyme: Glucocerebrosidase (β-glucosidase)
- Substrate: Glucocerebroside
⭐ Exam Favorite: Gaucher cells are described as "lipid-laden macrophages resembling crumpled tissue paper."
Niemann-Pick Disease
- Enzyme: Sphingomyelinase
- Substrate: Sphingomyelin
- 📌 Mnemonic: "No man PICKS his nose with his SPHINGer"

Sphingolipidosis metabolic pathway with enzyme deficiencies

Clinical Vignettes - Patient Presentations

Gaucher Disease: Ashkenazi Jewish child presents with massive splenomegaly, bone pain (aseptic necrosis of femur), and pancytopenia. Biopsy reveals "crinkled tissue paper" macrophages.
Niemann-Pick Disease: Infant with failure to thrive, progressive neurodegeneration, hepatosplenomegaly, and a macular "cherry-red spot." Histology shows lipid-laden "foam cells."
Tay-Sachs Disease: Infant of Ashkenazi Jewish descent with developmental regression, an exaggerated startle response (hyperacusis), and a "cherry-red spot." Critically, NO hepatosplenomegaly.
- 📌 Mnemonic: Tay-SaX lacks heXosaminidase A.
Fabry Disease (X-linked): Young male complains of episodic burning pain in hands/feet (peripheral neuropathy), develops angiokeratomas, and cannot sweat (hypohidrosis). Prone to renal failure.
Krabbe Disease: Infant (<6 months old) with extreme irritability, stiffness, peripheral neuropathy, and vision loss. Biopsy shows globoid cells.

⭐ The presence of hepatosplenomegaly is key to distinguishing Niemann-Pick from Tay-Sachs disease, as both present with a "cherry-red spot" and neurodegeneration.

High‑Yield Points - ⚡ Biggest Takeaways

Gaucher disease: Most common. Glucocerebrosidase deficiency causes hepatosplenomegaly, pancytopenia, and bone crises.

Tay-Sachs vs. Niemann-Pick: Both have "cherry-red" spots. Hepatosplenomegaly is absent in Tay-Sachs (Hexosaminidase A def.).

Fabry disease: X-linked. α-galactosidase A deficiency causes peripheral neuropathy, angiokeratomas, and renal failure.

Krabbe disease: Galactocerebrosidase deficiency causes peripheral neuropathy, optic atrophy, and globoid cells.

Metachromatic leukodystrophy: Arylsulfatase A deficiency causes demyelination.

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Lipid storage diseases