Correct Answer: A. Decreased synaptic transmission at thee myoneural junction
Myasthenia gravis (MG) is an autoimmune disorder characterized by decreased synaptic transmission at the neuromuscular junction (NMJ). The pathophysiology involves autoantibodies against nicotinic acetylcholine receptors (AChR) at the motor endplate, which leads to complement-mediated destruction and receptor blockade. This results in reduced available AChR for acetylcholine (ACh) binding, impairing signal transmission from nerve to muscle. Approximately 80–90% of generalized MG cases have anti-AChR antibodies; seronegative cases may have antibodies against muscle-specific kinase (MuSK) or LRP4, but the mechanism remains NMJ dysfunction. The clinical hallmark—fatigable weakness that worsens with repetitive activity—directly reflects this impaired neuromuscular transmission. Electromyography shows decremental response on repetitive nerve stimulation, confirming NMJ pathology. Single-fiber electromyography demonstrates increased "jitter" and blocking. Indian guidelines and standard neurology texts (Harrison, Robbins) emphasize that MG is fundamentally a disorder of receptor availability and signal transmission, not muscle fiber structure or contractile protein synthesis. Anticholinesterase agents (pyridostigmine) and immunosuppression (corticosteroids, azathioprine) are the mainstays of treatment in Indian clinical practice, targeting the NMJ dysfunction.
Why the other options are wrong
B. Absence of troponin C — This is wrong because troponin C is a contractile protein in the muscle fiber itself, not involved in neuromuscular transmission. Troponin deficiency would cause a primary myopathy with structural muscle weakness, not the characteristic fatigable weakness of MG. MG is a disorder of the NMJ, not of muscle fiber contractile apparatus. This option confuses muscle fiber pathology with NMJ dysfunction. C. Decreased myosin — This is wrong because myosin is a thick-filament contractile protein; decreased myosin would cause a primary myopathy (like nemaline myopathy or myosin storage disease), not MG. MG does not involve quantitative or qualitative defects in myosin. The weakness in MG is functional (due to impaired transmission) rather than structural. This option represents confusion between NMJ disorders and intrinsic muscle diseases. D. Decreased acetylcholine release at the nerve endings — This is wrong because MG involves normal or near-normal ACh release from the presynaptic terminal; the problem is on the postsynaptic side (AChR loss/blockade). Decreased ACh release occurs in Lambert-Eaton myasthenic syndrome (LEMS), which is caused by anti-calcium channel antibodies affecting presynaptic ACh mobilization. This is a classic NBE trap—students confuse MG with LEMS. MG is a postsynaptic disorder; LEMS is presynaptic.
High-Yield Facts
- Postsynaptic NMJ disorder: MG involves anti-AChR antibodies causing receptor loss/blockade, not presynaptic ACh release defect (unlike LEMS).
- Fatigable weakness is the cardinal clinical sign—worsens with repetitive activity, improves with rest; reflects progressive NMJ transmission failure.
- Decremental response on repetitive nerve stimulation (RNS) and increased jitter on single-fiber EMG are diagnostic electrophysiological hallmarks.
- Anti-AChR antibodies present in ~85% of generalized MG; seronegative cases may have anti-MuSK or anti-LRP4 antibodies.
- Anticholinesterase agents (pyridostigmine) and immunosuppression (prednisolone, azathioprine) are first-line treatments in Indian practice; thymectomy considered in thymoma-associated cases.
- Ocular MG (ptosis, diplopia) is the most common presentation in Indian patients; ~50% progress to generalized form within 2 years if untreated.
Mnemonics
MG vs LEMS (Post vs Pre) MG = Postsynaptic (AChR loss) → Muscle weakness, Gets worse with use. LEMS = Presynaptic (Ca²⁺ channels) → Less ACh release, Eases with use (warm-up phenomenon). Use: Differentiating two autoimmune NMJ disorders on exam. MG Antibodies: AChR > MuSK > LRP4 Anti-AChR (85% generalized, 50% ocular), Anti-MuSK (~40% seronegative), Anti-LRP4 (rare). Use: Remembering seropositive vs seronegative MG and which antibody to test first.
NBE Trap
NBE pairs MG with "decreased ACh release" to lure students who confuse it with LEMS (Lambert-Eaton). The key discriminator: MG is postsynaptic (AChR loss), LEMS is presynaptic (impaired ACh mobilization). Students who memorize "MG = NMJ problem" without understanding the site often pick the presynaptic option.
Clinical Pearl
In Indian clinical practice, ocular MG (ptosis and diplopia) is the most common presentation, especially in young women. A patient with isolated ptosis that worsens toward evening and improves after rest, combined with a positive edrophonium (Tensilon) test or anti-AChR serology, clinches the diagnosis. Early recognition and immunosuppression prevent progression to life-threatening generalized MG with respiratory muscle involvement.
_Reference: Harrison Ch. 382 (Myasthenia Gravis); Robbins Ch. 27 (Neuromuscular Disorders)_