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 15-year-old boy is sent from gym class with a chief complaint of severe muscle aches. In class today he was competing with his friends and therefore engaged in weightlifting for the first time. A few hours later he was extremely sore and found that his urine was red when he went to urinate. This concerned him and he was sent to the emergency department for evaluation. Upon further questioning, you learn that since childhood he has always had muscle cramps with exercise. Physical exam was unremarkable. Upon testing, his creatine kinase level was elevated and his urinalysis was negative for blood and positive for myoglobin. Thinking back to biochemistry you suspect that he may be suffering from a hereditary glycogen disorder. Given this suspicion, what would you expect to find upon examination of his cells?

Accumulation of glycogen in lysosomes forming dense granules

Glycogen without normal branching pattern

A 47-year-old woman presents to her primary care physician because of pain on urination, urinary urgency, and urinary frequency for 4 days. This is the third time for her to have these symptoms over the past 7 months. She was recently treated for candidal intertrigo. Vital signs reveal a temperature of 36.7°C (98.0°F), blood pressure of 110/70 mm Hg and pulse of 75/min. Physical examination is unremarkable except for morbid obesity. Her father has type 2 diabetes complicated by end-stage chronic kidney disease. A1C is found to be 8.5%. The patient is given a prescription for her urinary symptoms. Which of the following is the best next step for this patient?

Sulphonylurea added to metformin

An 8-month-old female infant from a first-degree consanguineous couple was brought to the physician because the mother noticed abnormalities in the growth of her child as well as the different lengths of her child's legs. The infant had gingival hyperplasia, restricted movement in both shoulders, a prominent, pointed forehead, and enophthalmos with a slight opacity in both corneas. A blood test revealed 10 fold higher than normal levels of the following enzymes: N-acetyl-ß-glucosaminidase, ß-glucuronidase, ß-hexosaminidase A, and alkaline phosphatase. Which of the following is most likely deficient in this patient?

N-acetyl-glucosamine-1-phosphotransferase

Lysosomal alpha-1,4-glucosidase

Glucose-6-phosphate dehydrogenase

A 12-year-old boy is brought to the physician for a well-child examination. He feels well. He has no history of serious illness. He has received all age-appropriate screenings and immunizations. His 7-year-old brother was treated for nephrotic syndrome 1 year ago. He is at 50th percentile for height and 60th percentile for weight. His temperature is 37°C (98.6°F), pulse is 90/min, and blood pressure is 96/54 mm Hg. Physical examination shows no abnormalities. Urine dipstick shows 1+ protein. A subsequent urinalysis of an early morning sample shows: Blood negative Glucose negative Protein trace Leukocyte esterase negative Nitrite negative RBC none WBC 0–1/hpf Protein/creatinine ratio 0.2 (N ≤ 0.2) Which of the following is the most appropriate next step in management?

Repeat urine dipstick in 1 year

Measure serum creatinine and urea nitrogen

24-hour urine protein collection

A 13-year-old girl is brought to the physician by her mother because of a 1-month history of abnormal movements of her muscles that she cannot control. She has a younger brother with cognitive disabilities and epilepsy. Examination shows frequent, brief, involuntary contractions of the muscle groups of the upper arms, legs, and face that can be triggered by touch. An EEG shows generalized epileptiform activity. A trichrome stain of a skeletal muscle biopsy specimen shows muscle fibers with peripheral red inclusions that disrupt the normal fiber contour. Which of the following is the most likely underlying mechanism of the patient's symptoms?

Defective oxidative phosphorylation

CTG trinucleotide repeat expansion

Mutation of the methyl-CpG binding protein 2 gene

Autoimmune endomysial destruction

An 8-year-old boy presents with recurrent infections including multiple episodes of pneumonia and diarrhea. He reports difficulty seeing in the dark. Physical examination reveals white patches on the sclera and conjunctival dryness. What is the most likely cause of these findings?

Spinocerebellar ataxia (SCA) type 1

Glycoproteinoses for USMLE Step 2 CK: High-Yield Biochemistry Lessons

Glycoproteinoses - Sugar-Coated Chaos

Lysosomal storage disorders caused by deficient glycoprotein-degrading enzymes, leading to the accumulation of oligosaccharides with attached peptide fragments.

Angiokeratomas in Fucosidosis: Clinical and Histological

Aspartylglucosaminuria:
- Defect: Aspartylglucosaminidase (AGA).
- Features: Progressive neurodegeneration, coarse facies, connective tissue abnormalities, intellectual disability starting at age 2-4.
Fucosidosis:
- Defect: α-L-Fucosidase (FUCA1).
- Features: Coarse facies, growth retardation, psychomotor retardation, visceromegaly.

⭐ Angiokeratomas are a key feature of Fucosidosis, similar to Fabry disease, but Fucosidosis presents with coarser facial features and intellectual disability.

The Disorders - Rogues' Gallery

Glycoproteinoses are a group of lysosomal storage disorders caused by defects in glycoprotein degradation. This leads to the accumulation of oligosaccharides and glycopeptides in lysosomes, resulting in multisystemic clinical manifestations.

Disorder	Deficient Enzyme	Accumulated Substrate	Key Clinical Features
Aspartylglucosaminuria	Aspartylglucosaminidase	Aspartylglucosamine	Late-onset coarse facies, intellectual disability, angiokeratomas, macroglossia.
Fucosidosis	α-L-Fucosidase	Fucose-containing glycolipids & oligosaccharides	Coarse facies, growth retardation, dysostosis multiplex, angiokeratomas, ↑ sweat salinity.
Schindler Disease	α-N-Acetylgalactosaminidase (α-NAGA)	Glycopeptides with α-N-acetylgalactosaminyl termini	Type I (infantile): neuroaxonal dystrophy, rapid neurodegeneration. Type II (adult): milder, angiokeratoma corporis diffusum.
Sialidosis (Mucolipidosis I)	α-N-Acetylneuraminidase (Sialidase)	Sialyloligosaccharides	Type I: normal intelligence, myoclonus, ataxia, cherry-red spot. Type II: infantile onset, coarse facies, dysostosis multiplex.

⭐ Exam Favourite: The presence of a macular cherry-red spot on fundoscopy is a classic finding in Sialidosis (Type I), similar to Tay-Sachs disease, but distinguished by the associated myoclonus and normal early development.

Diagnosis & Management - The Clinician's Playbook

Diagnosis
- Screening: Urinary oligosaccharide analysis.
- Definitive Diagnosis: Enzyme activity assays in cultured fibroblasts or leukocytes.
- Confirmation: Molecular genetic testing for specific gene mutations.
Management
- Primary Approach: Symptomatic and supportive care.
  - Managing seizures, recurrent infections, and skeletal abnormalities.
- Therapeutic Options:
  - Hematopoietic stem cell transplantation (HSCT) has shown limited success in some cases (e.g., fucosidosis).
  - Enzyme replacement therapy (ERT) and chaperone therapies are largely investigational.

⭐ Exam Favorite: Aspartylglucosaminuria has a significantly high prevalence in the Finnish population due to a founder effect.

High‑Yield Points - ⚡ Biggest Takeaways

Glycoproteinoses are a group of autosomal recessive disorders resulting from defects in glycoprotein catabolism.

This leads to lysosomal accumulation of glycans or glycopeptides, causing widespread cellular dysfunction.

Clinical overlap with MPS is significant: coarse facial features, dysostosis multiplex, and psychomotor retardation.

Distinguishing features can include angiokeratomas, hearing loss, and myoclonus.

Diagnosis is confirmed by detecting oligosaccharides in urine and through specific enzyme activity assays.

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