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Myasthenia gravis is an autoimmune disorder caused by antibodies against postsynaptic acetylcholine receptors at the neuromuscular junction. It presents with fluctuating muscle weakness, worsening with activity and improving with rest. Common features include diplopia, ptosis, dysphagia, and generalized weakness. Lambert–Eaton syndrome involves presynaptic calcium channel antibodies, associated with small cell lung carcinoma, and improves with activity. Multiple sclerosis causes demyelination in the CNS with optic neuritis, motor and sensory deficits. Guillain–Barré syndrome is an acute demyelinating polyneuropathy with ascending weakness. Myasthenia gravis is associated with thymoma or thymic hyperplasia. Treatment includes acetylcholinesterase inhibitors (pyridostigmine), immunosuppression, and thymectomy in select patients.

Multiple myeloma is a malignant proliferation of plasma cells producing monoclonal immunoglobulins or light chains. Classic features include CRAB: hyperCalcemia, Renal dysfunction, Anemia, and Bone lesions (lytic “punched-out” lesions on X-ray). The monoclonal protein spike (M-protein) is seen on serum protein electrophoresis, and urine may reveal Bence Jones proteins. Renal failure is a common complication, often due to light chain cast nephropathy causing tubular obstruction and interstitial inflammation. Nephrolithiasis is less typical. Amyloidosis (AL type) can occur but is less common than nephropathy. Osteophytes occur in osteoarthritis, not multiple myeloma. Management includes chemotherapy (bortezomib, lenalidomide, dexamethasone), bisphosphonates for bone disease, and stem cell transplant in eligible patients. Prognosis varies depending on cytogenetics and response to therapy.

Benign prostatic hyperplasia (BPH) is a common condition in older men due to hyperplasia of periurethral (transition zone) prostate tissue. It causes lower urinary tract symptoms (hesitancy, weak stream, nocturia, incomplete emptying). DRE reveals a smooth, symmetrically enlarged prostate. Prostate adenocarcinoma typically involves the peripheral zone and feels firm or nodular. Bladder carcinoma presents with painless hematuria, not obstructive voiding. Urethral strictures may follow trauma or infection but are less common. Management includes lifestyle modification, alpha-adrenergic antagonists (tamsulosin), 5-alpha-reductase inhibitors (finasteride), or surgery (TURP). BPH is benign but impacts quality of life significantly if untreated.

Transitional cell carcinoma (urothelial carcinoma) is the most common type of bladder cancer. Risk factors include smoking, aniline dye exposure, cyclophosphamide, and phenacetin use. Patients present with painless hematuria, frequency, and urgency. Gross hematuria is often the earliest sign. Renal cell carcinoma originates in the kidney and presents with flank pain, hematuria, and abdominal mass. Squamous cell carcinoma of the bladder is rare in developed countries and associated with chronic irritation (schistosomiasis, catheters). Prostate carcinoma arises from the prostate and causes urinary obstruction and bone metastases, not bladder mass. Management involves surgical resection, intravesical therapy (BCG), and systemic therapy in advanced cases.

Acanthosis nigricans presents with hyperpigmented, velvety plaques in intertriginous areas (neck, axillae, groin). It is associated with insulin resistance (obesity, type 2 diabetes, PCOS) and, less commonly, gastric adenocarcinoma. Psoriasis causes erythematous plaques with silvery scale. Seborrheic keratosis presents as benign waxy, stuck-on lesions. Lichen planus presents with pruritic, polygonal, purple papules with Wickham striae. Management of acanthosis nigricans focuses on treating the underlying condition (weight loss, insulin resistance). If associated with sudden onset and extensive involvement, evaluation for underlying malignancy is warranted.