The correct answer is C. Chronic kidney disease (CKD) staging is based on both the glomerular filtration rate (GFR) and albuminuria levels. The GFR stages are: G1 (>90), G2 (60-89), G3a (45-59), G3b (30-44), G4 (15-29), and G5 (<15). The albuminuria stages are: A1 (<30 mg/day), A2 (30-300 mg/day), and A3 (>300 mg/day). This patient's eGFR of 35 mL/min/1.73 m² places him in stage G3b. His albuminuria of 500 mg/day places him in stage A3. Therefore, his CKD classification is G3b A3.

The correct answer is B. The patient's symptoms and ECG findings (peaked T waves, widened QRS) are highly suggestive of severe hyperkalemia, a common complication of advanced CKD. The most important immediate step is to stabilize the cardiac membrane to prevent life-threatening arrhythmias. Intravenous calcium gluconate is the first-line agent for cardiac membrane stabilization and does not lower the serum potassium level. After stabilizing the membrane, therapies to shift potassium intracellularly (A - insulin and glucose) and remove it from the body (C, D) should be initiated. Urgent hemodialysis (D) is definitive treatment but is not the most immediate step. Sodium polystyrene sulfonate (C) is slow-acting and not appropriate for acute, life-threatening hyperkalemia.

The correct answer is B. This patient's GFR is approaching the level where renal replacement therapy (RRT) is typically initiated (<10-15 mL/min/1.73 m²). Preparation should begin well in advance. The preferred long-term access for hemodialysis is an arteriovenous (AV) fistula due to its lower rates of infection and thrombosis and superior longevity. Given his interest in RRT, referral for AV fistula creation is the most appropriate next step. Peritoneal dialysis (A) is a home-based option but is relatively contraindicated in this patient due to his history of multiple abdominal surgeries, which increases the risk of catheter dysfunction and peritonitis. A tunneled catheter (C) is a less preferred option for long-term access due to high infection rates and should only be used if an AV fistula or graft is not possible. Delaying planning (D) is inappropriate given his low GFR and early uremic symptoms.

The correct answer is C. Patients with CKD often develop a chronic metabolic acidosis due to the kidneys' inability to excrete the daily acid load. This chronic acidemia has several detrimental effects, most notably acting as a buffer system by mobilizing calcium carbonate from bone, leading to bone demineralization and worsening CKD-MBD. Correcting the acidosis with oral sodium bicarbonate can prevent this process. While treating acidosis has been associated with slowing CKD progression and may have other benefits, its most direct and well-established effect is on bone health. It does not directly improve potassium (A), although severe acidosis can worsen hyperkalemia. Effects on cardiovascular mortality (B) or hemoglobin (D) are less direct.

The correct answer is C. This patient's presentation is classic for dialysis-related amyloidosis, which is caused by the deposition of beta-2 microglobulin in osteoarticular structures. Beta-2 microglobulin is a medium-sized molecule that is not effectively cleared by conventional hemodialysis membranes, leading to its accumulation over years of treatment. It commonly affects the shoulders, hips, and wrists, causing arthropathy, bone cysts, and carpal tunnel syndrome. Calcium pyrophosphate (A) causes pseudogout. Monosodium urate (B) causes gout, which is common in CKD but has a different clinical and radiographic appearance. Hydroxyapatite (D) deposition can occur but is less likely to cause this specific syndrome.

The correct answer is C. This patient is presenting with signs and symptoms of severe uremia, including encephalopathy (altered mental status) and pericarditis (pericardial friction rub). Uremic pericarditis is an absolute indication for the initiation of renal replacement therapy. Urgent hemodialysis is required to remove the uremic toxins and prevent progression to cardiac tamponade. High-dose NSAIDs (A) or corticosteroids (D) are used for viral or idiopathic pericarditis but are ineffective and potentially harmful (NSAIDs are nephrotoxic) in uremic pericarditis. Pericardiocentesis (B) is only indicated if there is evidence of cardiac tamponade (e.g., hypotension, pulsus paradoxus, jugular venous distention), which is not described here.

The correct answer is B. In patients with CKD and known vascular or soft tissue calcification, non-calcium-based phosphate binders are preferred over calcium-based binders. Sevelamer (a non-absorbable polymer) and lanthanum are first-line non-calcium-based options. Calcium-based binders like calcium carbonate (A) can contribute to a positive calcium balance and may worsen vascular calcification. Aluminum hydroxide (C) is a potent phosphate binder but is avoided for long-term use due to the risk of aluminum toxicity (adynamic bone disease, encephalopathy). Magnesium-containing binders (D) are generally avoided in CKD due to the risk of hypermagnesemia.

The correct answer is D. The decision to initiate dialysis is primarily a clinical one, based on the presence of signs and symptoms of uremia that are refractory to medical management, rather than a specific GFR number. This patient's significant fatigue, anorexia, and weight loss (failure to thrive) are severe uremic symptoms that constitute a clear indication to begin dialysis. While most patients start dialysis with an eGFR between 5-15 mL/min/1.73 m² (A), the GFR value itself is not an absolute indication. The presence of cysts (B) is the underlying disease, not an indication for dialysis. His blood pressure is controlled (C).

The correct answer is D. This patient's clinical picture is consistent with obstructive uropathy secondary to BPH. Chronic obstruction of urine outflow leads to increased pressure in the ureters and renal pelves (hydronephrosis). This pressure is transmitted proximally to the Bowman's space within the glomerulus. The elevated hydrostatic pressure in Bowman's space opposes the pressure driving glomerular filtration, leading to a decrease in GFR. Over time, this sustained pressure causes tubulointerstitial fibrosis and progressive chronic kidney disease. Glomerular deposition (A), hypertensive injury (B), and primary tubulointerstitial inflammation (C) are other causes of CKD but do not fit the obstructive picture presented.

The correct answer is C. Anemia of chronic kidney disease is primarily caused by decreased production of erythropoietin by the failing kidneys. Before initiating therapy with an erythropoiesis-stimulating agent (ESA), it is essential to ensure adequate iron stores, as iron is a necessary substrate for erythropoiesis. This patient's ferritin (>100 ng/mL) and TSAT (>20%) indicate adequate iron stores. Therefore, the most appropriate next step is to initiate an ESA like epoetin alfa or darbepoetin alfa. Oral iron (A) is not indicated as his stores are replete. Blood transfusion (B) is reserved for severe, symptomatic anemia or acute hemorrhage. A bone marrow biopsy (D) is not indicated as the cause of anemia is clear.