A 25-year-old woman presents with a complaint of fatigue and muscle weakness. On examination, she has ptosis and weakness of the proximal muscles of the upper limbs. Her vital signs are stable, and laboratory tests reveal a positive anti-acetylcholine receptor antibody test. Which of the following is the most likely diagnosis?

Correct Answer: B

Rationale: The correct answer is B: Myasthenia gravis. This autoimmune disorder is characterized by muscle weakness and fatigability due to antibodies attacking acetylcholine receptors at the neuromuscular junction. The presence of anti-acetylcholine receptor antibodies in this patient supports the diagnosis. Ptosis and weakness of proximal muscles are classic features. Choice A, Multiple sclerosis, is a central nervous system disorder and typically presents with sensory and motor deficits, not ptosis or fluctuating weakness. Guillain-Barré syndrome (Choice C) is characterized by ascending weakness and is typically preceded by an infection, unlike the chronic presentation in myasthenia gravis. Lambert-Eaton syndrome (Choice D) involves antibodies against presynaptic calcium channels and presents with proximal muscle weakness but often includes autonomic symptoms, which are not evident in this patient.

Correct Answer: C

Rationale: The most likely diagnosis is breast cancer recurrence (Choice C) due to the patient's history of breast cancer and the characteristics of the mass (firm, non-tender, fixed). Recurrence is common in breast cancer survivors. Fibrocystic changes (Choice A) typically present with lumpy or tender breasts and are not usually fixed. Breast abscess (Choice B) would present with more acute symptoms like redness, warmth, and fever. Fat necrosis (Choice D) may mimic a cancerous mass but is usually not fixed to underlying tissue. In this case, the history and characteristics of the mass strongly suggest breast cancer recurrence.

Correct Answer: B

Rationale: The correct answer is B: Hypertensive crisis. The patient's symptoms of headache, blurred vision, and nausea, along with severely elevated blood pressure (200/120 mm Hg), suggest a hypertensive emergency. In this situation, the elevated blood pressure is causing end-organ damage, leading to symptoms. Treatment is urgent to prevent further complications. Choice A: Primary hypertension is unlikely as the sudden onset of severe symptoms and extremely high blood pressure indicate an acute issue rather than chronic primary hypertension. Choice C: Migraine typically presents with specific symptoms like throbbing head pain, sensitivity to light or sound, and may have an aura. The lack of these symptoms and the presence of severely elevated blood pressure make migraine less likely. Choice D: Cluster headaches are characterized by severe, unilateral head pain with associated symptoms like eye redness or tearing. The lack of these specific symptoms and the presence of markedly elevated blood pressure make cluster headache less likely.

Correct Answer: B

Rationale: The correct answer is B: Cool, pale skin. Arterial insufficiency results in decreased blood flow to the extremities, leading to reduced skin temperature and pallor. Cool skin indicates diminished blood supply, while pale skin suggests vasoconstriction. Pitting edema (choice A) is a sign of venous insufficiency, not arterial. Brown discoloration (choice C) is characteristic of chronic venous insufficiency due to hemosiderin deposition. Warm, erythematous skin (choice D) is a sign of inflammation or infection, not arterial insufficiency.

Correct Answer: A

Rationale: The correct answer is A: Cerebellar dysfunction. When a patient sways significantly and loses balance while walking in a straight line, it suggests dysfunction of the cerebellum. The cerebellum is responsible for coordination, balance, and fine motor control. Impairment in the cerebellum can lead to ataxia, which is characterized by uncoordinated movements and difficulty maintaining balance. Vestibular impairment (B) primarily affects the inner ear's balance system, not coordination of movement. Motor weakness (C) typically presents as muscle weakness and affects strength, not balance. Peripheral neuropathy (D) involves damage to peripheral nerves, leading to sensory and motor deficits, but not specifically related to coordination and balance issues like cerebellar dysfunction.