Basic Science and Diagnostic Imaging
This domain tests applied physiology, pathology, imaging selection and the statistics needed to interpret a test. Questions often combine a clinical scenario with a numerical or anatomical answer, so precision matters more than breadth.
Physiology and pharmacology
- GFR is best estimated with creatinine-based equations, but creatinine depends on muscle mass, so it overestimates GFR in sarcopenic and elderly patients. Cystatin C is useful when muscle mass is atypical.
- Know the receptor targets: alpha-1A for prostatic smooth muscle, M3 for detrusor contraction, beta-3 for detrusor relaxation, and the renin-angiotensin axis for renovascular hypertension.
- Renal concentrating ability and potassium handling explain why obstructive uropathy causes hyperkalaemia and acidosis.
Imaging selection
- Ultrasound first for hydronephrosis, bladder volume and paediatric assessment. Non-contrast CT for stones and trauma. CT urography for painless haematuria and upper tract urothelial carcinoma, with a split-bolus technique reducing radiation.
- Multiparametric MRI is reported with PI-RADS for prostate cancer and VI-RADS for bladder cancer invasion. Bosniak classification guides renal cyst management: I and II are benign, IIF requires follow-up, III and IV require surgery.
- Nuclear medicine: MAG3 for function and drainage, DMSA for cortical scarring, bone scan and PSMA PET for staging.
- Radiation: CT urography delivers roughly 10-15 mSv, so justify every examination.
Tumour markers and statistics
- PSA is organ-specific, not cancer-specific. hCG has a half-life of 24-36 hours, AFP of 5-7 days, which governs how soon after orchidectomy markers are rechecked.
- Sensitivity, specificity, positive and negative predictive values, and likelihood ratios are all fair game.
Worked example
A screening cohort has 10 percent prevalence of significant prostate cancer. A PSA threshold has 80 percent sensitivity and 60 percent specificity. Positive predictive value equals (0.8 x 0.1) divided by [(0.8 x 0.1) + (0.4 x 0.9)] = 0.08 / 0.44, about 18 percent. Negative predictive value equals (0.9 x 0.6) divided by [(0.9 x 0.6) + (0.2 x 0.1)] = 0.54 / 0.56, about 96 percent. So a positive result is weak evidence in a low-prevalence population, while a negative result is reassuring.
Traps that cost marks
Equating a normal creatinine with normal GFR; treating PI-RADS 3 as cancer; misclassifying a Bosniak IIF cyst as surgical; forgetting hCG can rise with hypogonadism or marijuana use; confounding prevalence-dependent predictive values with sensitivity.