A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 10-year-old boy is brought to the physician because of recurring episodes of achy muscle pain in his legs. He has a history of poor school performance despite tutoring and has been held back two grades. He is at the 40th percentile for height and 30th percentile for weight. Examination shows ptosis, a high-arched palate, and muscle weakness in the face and hands; muscle strength of the quadriceps and hamstrings is normal. Sensation is intact. Percussion of the thenar eminence causes the thumb to abduct and then relax slowly. Which of the following is the most likely underlying cause?
-
A
Apoptosis of lower motor neuronsIncorrect. Apoptosis of lower motor neurons (SMA) causes proximal limb weakness without myotonia.
-
B
Complete impairment of the dystrophin proteinIncorrect. Complete dystrophin loss (Duchenne) causes pseudohypertrophy and Gowers sign without myotonia or intellectual disability of this character.
-
C
Humoral immune attack against the endomysial blood vesselsIncorrect. Humoral attack on endomysial vessels describes dermatomyositis with heliotrope rash and Gottron papules, not myotonia.
-
D
CTG trinucleotide expansion in the DMPK geneCorrect. CTG trinucleotide expansion in DMPK causes myotonic dystrophy type 1 — explaining percussion myotonia, distal weakness, cataracts, and multisystem involvement.
-
E
Autoantibodies against the postsynaptic acetylcholine receptorIncorrect. Anti-AChR antibodies cause myasthenia gravis with fatigable proximal/ocular weakness that improves with rest, not the percussion myotonia and CTG-repeat-driven multisystem features described.
↑ Tap an answer to reveal the reasoning
Answer: D. A 10-year-old boy with multisystem disease — facial muscle weakness, ptosis, high-arched palate, hand muscle weakness, intellectual disability (academic struggles, held back two grades), and percussion myotonia (thenar contraction with delayed relaxation) — has myotonic dystrophy type 1 (DM1). The cardinal feature is grip and percussion myotonia (sustained muscle contraction with delayed relaxation, also seen as 'difficulty releasing handshake').
DM1 is caused by a CTG trinucleotide repeat expansion in the 3' untranslated region of the DMPK (dystrophia myotonica protein kinase) gene on chromosome 19. The expanded CTG repeats produce toxic mRNA that sequesters splicing factors (MBNL1), causing dysregulated alternative splicing of multiple genes — including the muscle chloride channel CLCN1 (myotonia), insulin receptor (insulin resistance), and cardiac troponin T. DM1 shows anticipation — earlier onset and worsening severity in successive generations as the repeat expands.
Multisystem features include cataracts (Christmas tree appearance), frontal balding, testicular atrophy, cardiac conduction defects, insulin resistance/diabetes, and intellectual disability.
Lower motor neuron apoptosis describes spinal muscular atrophy (SMN1 mutation) — proximal weakness without myotonia. Complete dystrophin loss is Duchenne muscular dystrophy — pseudohypertrophy, Gowers sign, no myotonia. Humoral attack on endomysial vessels describes dermatomyositis — heliotrope rash, Gottron papules, proximal weakness, no myotonia.
**Why each option:**
**A.** Apoptosis of lower motor neurons (SMA) causes proximal limb weakness without myotonia.
**B.** Complete dystrophin loss (Duchenne) causes pseudohypertrophy and Gowers sign without myotonia or intellectual disability of this character.
**C.** Humoral attack on endomysial vessels describes dermatomyositis with heliotrope rash and Gottron papules, not myotonia.
**D.** CTG trinucleotide expansion in DMPK causes myotonic dystrophy type 1 — explaining percussion myotonia, distal weakness, cataracts, and multisystem involvement.