What enzyme is most likely deficient in a man with reduced sperm motility and an unusually high concentration of an osmotically active compound in his seminal fluid, hinting at a possible enzymatic deficiency that prevents conversion to fructose?

Differential Diagnosis for Enzyme Deficiency in Seminal Fluid

The question presents a scenario where a man has reduced sperm motility, and a metabolic profile of his seminal fluid shows an unusually high concentration of an osmotically active compound, suggesting an enzymatic deficiency that prevents the conversion of this compound to fructose. Based on this information, we can consider the following differential diagnoses:

• Single most likely diagnosis
  • Aldolase B: This enzyme is involved in the breakdown of fructose-1-phosphate to glyceraldehyde and dihydroxyacetone phosphate. A deficiency in aldolase B could lead to an accumulation of fructose-1-phosphate, which is an osmotically active compound. This accumulation could be related to the reduced sperm motility observed in the patient.
• Other Likely diagnoses
  • Fructokinase: This enzyme phosphorylates fructose to form fructose-1-phosphate, which is then further metabolized by aldolase B. A deficiency in fructokinase could lead to an accumulation of fructose, which is also an osmotically active compound.
• Do Not Miss
  • None: In this scenario, the key issue is identifying the enzyme deficiency that could lead to the accumulation of an osmotically active compound. While other enzyme deficiencies could potentially cause similar issues, the information provided points towards a problem with fructose metabolism.
• Rare diagnoses
  • Aldose reductase: This enzyme is involved in the polyol pathway, where glucose is converted to sorbitol. While a deficiency in aldose reductase could lead to issues with osmotically active compounds, it is less directly related to fructose metabolism than aldolase B or fructokinase.
  • Sorbitol dehydrogenase: This enzyme is also involved in the polyol pathway, converting sorbitol to fructose. A deficiency could potentially lead to an accumulation of sorbitol, an osmotically active compound, but it is less likely given the specific hint towards a problem with fructose conversion.
  • Hexokinase: This enzyme phosphorylates hexoses (six-carbon sugars) to form hexose phosphate. While important in glucose metabolism, a deficiency in hexokinase is less directly related to the accumulation of an osmotically active compound in the context of fructose metabolism as described.