[14.2] A postoperative patient on intravenous fluids develops lesions in the
mouth (angular stomatitis). Urinalysis indicates an excretion of 15 pg
riboflavin/mg creatinine, which is abnormally low. Which of the following
TCA cycle enzymes is most likely to be affected?
A. Citrate synthase
B. Isocitrate dehydrogenase
D. Malate dehydrogenase
E. Succinate dehydrogenase
[14.3] After excessive drinking over an extended period of time while eating
poorly, a middle-aged man is admitted to the hospital with “high out-
put” heart failure. Which of the following enzymes is most likely
B. Citrate synthase
C. Isocitrate dehydrogenase
D. a-Ketoglutarate dehydrogenase
E. Succinate thiokinase
A nsw ers
A. After 4 hours of heavy exercise, glycogen stored in muscle cells
has been expended. Free glycerol cannot be used by the muscle cell
because it does not have the enzyme (glycerol kinase) that will phos-
phorylate it so that it can enter the glycolytic pathway. Although glu-
cose and ketone bodies can be taken up by the muscle cells and used
for energy, fatty acid oxidation provides most of the ATP for the
marathon runner at this point in the race.
E. The patient has demonstrated a deficiency in riboflavin (urinary
excretion of less than 30 pg/mg creatinine is considered clinically
deficient). Riboflavin is a component of the cofactor FAD (flavin
adenine dinucleotide), which is required for the conversion of succi-
nate to fumarate by succinate dehydrogenase.
D. This patient has exhibited symptoms of beri beri heart disease,
which is a result of a nutritional deficiency in vitamin B1 (thiamine).
The active form of the vitamin, thiamine pyrophosphate, is a required
cofactor for a-ketoglutarate dehydrogenase.