Foundational Science and Mechanisms of Disease

A 54-year-old woman comes to the clinic due to persistent joint pain and morning stiffness. She was diagnosed with rheumatoid arthritis two years ago and has been taking oral methotrexate, but her symptoms remain poorly controlled. Physical examination shows active synovitis and effusion in the bilateral metacarpophalangeal and proximal interphalangeal joints. The physician plans to initiate a tumor necrosis factor (TNF) alpha inhibitor. Before starting the new medication, an interferon-gamma release assay is obtained and is positive. A chest radiograph shows no abnormalities, and she has no cough, night sweats, or weight loss. The physician decides to delay the biologic therapy and initiates a course of rifampin. The proposed biologic therapy increases the risk of disease reactivation in this patient by directly impairing which of the following immunologic processes?

A $64$-year-old woman is evaluated in the oncology clinic to discuss systemic therapy for newly diagnosed metastatic colorectal cancer. Two weeks ago, she underwent a right hemicolectomy for an obstructing cecal mass. Intraoperative findings and subsequent imaging confirmed the presence of unresectable bilobar hepatic metastases. Pathology of the primary tumor demonstrated moderately differentiated adenocarcinoma. Comprehensive molecular profiling of the tumor tissue reveals a missense mutation in codon $12$ of the KRAS gene. The patient has no other significant past medical history and her Eastern Cooperative Oncology Group (ECOG) performance status is $0$. The medical team plans to initiate a combination cytotoxic chemotherapy regimen. Based on the molecular pathology findings, the addition of a monoclonal antibody directed against which of the following targets is specifically contraindicated due to lack of therapeutic benefit?

A 42-year-old man presents to the outpatient clinic with a 2-year history of worsening joint pain, excessive sweating, and an increase in his shoe size. On physical examination, he has a prominent brow, macroglossia, and enlarged, doughy hands. Visual field testing is normal. Laboratory evaluation reveals a markedly elevated serum insulin-like growth factor-1 (IGF-1) concentration. Following administration of a 75-gram oral glucose load, serum growth hormone (GH) levels remain elevated. The hormone directly responsible for the peripheral somatic changes observed in this patient exerts its cellular effects via which of the following intracellular signaling mechanisms?

An endocrinology consult team is evaluating laboratory panels for four different patients who have presented with perturbations in calcium and phosphate homeostasis. To establish the correct diagnosis and formulate a management plan, the underlying pathophysiology must be identified. Match each patient's specific biochemical profile with the precise cellular or metabolic mechanism responsible for the findings.

A 38-year-old man is evaluated in the nephrology clinic for a gradual increase in serum creatinine over the past two weeks. He underwent a deceased-donor renal transplantation 8 months ago for end-stage renal disease secondary to IgA nephropathy. His current medications include tacrolimus, mycophenolate mofetil, and prednisone. His blood pressure is 148/92 mmHg. Physical examination is unremarkable, and the patient reports no fever or dysuria. Laboratory studies show a serum creatinine of 2.6 mg/dL, an increase from his baseline of 1.3 mg/dL. A renal ultrasound reveals normal allograft size with no hydronephrosis and normal resistive indices. A renal allograft biopsy is performed. Immunofluorescence staining of the biopsy specimen reveals diffuse, strong linear deposition of C4d along the peritubular capillaries. Which of the following is the most likely initiating immunopathologic event responsible for this patient's current allograft dysfunction?

An emergency department physician evaluates a 34-year-old woman presenting with a 3-day history of continuous epistaxis, heavy menstrual bleeding, and diffuse ecchymoses. She reports profound fatigue over the past week but denies fever or weight loss. Her temperature is 37.1°C (98.8°F), blood pressure is 110/70 mmHg, and pulse is 105/min. Physical examination reveals widespread petechiae over her lower extremities.

Laboratory studies reveal the following:

A peripheral blood smear demonstrates numerous mononuclear cells with intensely granulated cytoplasm and prominent needle-like cytoplasmic inclusions.

Which of the following is the most appropriate initial step in the management of this patient's condition?